feat: add database accessory (#3)

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## What type of PR is this?

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/kind feature
## What this PR does / why we need it:
This PR adds support for database accessory which defines the relational
database service (rds) provided by a specified cloud vendor.
## Which issue(s) this PR fixes:

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## Special notes for your reviewer:
The implementation of Database Generator can be found at [feat: add
database generator](KusionStack/kusion#435)

### Does this PR introduce a user-facing change?

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### Additional documentation e.g., design docs, usage docs, etc.:

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```docs

```

models/schema/v1/accessories/database.k

@@ -0,0 +1,93 @@
+schema Database: 
+    """ As an important supporting accessory, Database describes the attributes
+    to locally deploy or create a cloud provider managed database instance for 
+    the workload. 
+
+    Attributes
+    ----------
+    type: str, default is Undefined, required. 
+        Type defines the local deployment mode or the specific cloud vendor that 
+        provides the relational database service (rds). 
+    engine: str, default is Undefined, required. 
+        Engine defines the database engine to use. 
+    version: str, default is Undefined, required. 
+        Version defines the database engine version to use. 
+    instanceType: str, default is Undefined, optional. 
+        InstanceType defines the type of the database which is required when 
+        creating an rds instance provided by the cloud vendor. 
+    size: int, default is 10, optional. 
+        Size defines the allocated storage size of the rds instance provided by 
+        the cloud vendor in GB.  
+    category: str, default is "Basic", optional. 
+        Category defines the edition of the rds instance provided by the cloud 
+        vendor. 
+    username: str, default is "root", optional. 
+        Username defines the operation account for the database. 
+    securityIPs: [str], default is ["0.0.0.0/0"], optional. 
+        SecurityIPs defines the list of IP addresses allowed to access the rds 
+        instance provided by the cloud vendor. 
+    subnetID: str, default is Undefined, optional. 
+        SubnetID defines the virtual subnet ID associated with the VPC that the rds 
+        instance will be created in. 
+    privateLink: bool, default is True, optional. 
+        PrivateLink defines whether the host address of the rds instance for the workload 
+        to connect with is via public network or private network of the cloud vendor. 
+    extraMap: {str:str}, default is Undefined, optional. 
+        ExtraMap defines the diversified rds configuration items from different
+        cloud vendors. 
+
+    Examples
+    --------
+    Instantiate an aws rds with mysql 5.7. 
+
+    import models.schema.v1.accessories.database as db
+
+    database: db.Database {
+        type: "aws"
+        engine: "mysql"
+        version: "5.7"
+        instanceType: "db.t3.micro"
+    }
+    """
+
+    # The local deployment mode or the specific cloud vendor that provides the 
+    # relational database service (rds). 
+    type:              str
+
+    # The database engine to use. 
+    engine:             str
+
+    # The database engine version to use. 
+    version:            str
+
+    # The type of the database which is required when creating an rds instance
+    # provided by the cloud vendor. 
+    instanceType?:       str
+
+    # The allocated storage size of the rds instance provided by the cloud vendor 
+    # in GB. 
+    size?:              int = 10
+
+    # The edition of the rds instance provided by the cloud vendor. 
+    category?:          str = "Basic"
+
+    # The operation account for the database. 
+    username?:          str = "root"
+
+    # The list of IP addresses allowed to access the rds instance provided by the 
+    # cloud vendor. 
+    securityIPs?:       [str] = ["0.0.0.0/0"]
+
+    # The virtual subnet ID associated with the VPC that the rds instance will be
+    # created in. 
+    subnetID?: str
+
+    # Whether the host address of the rds instance for the workload to connect with 
+    # is via public network or private network of the cloud vendor. 
+    privateLink?:    bool = True
+
+    # The diversified rds configuration items from different cloud vendors. 
+    extraMap?:          {str:str}
+
+    check: 
+        instanceType if type != "local", "instanceType is required for cloud provider managed database"

models/schema/v1/app_configuration.k

@@ -1,5 +1,6 @@
 import models.schema.v1.workload as wl
 import models.schema.v1.trait as t
+import models.schema.v1.accessories.database as db
 
 schema AppConfiguration:
     """ AppConfiguration is a developer-centric definition that describes how to run an Application.
@@ -39,6 +40,9 @@ schema AppConfiguration:
     # OpsRule specifies collection of rules that will be checked for Day-2 operation.
     opsRule?:                   t.OpsRule
 
+    # Database describes a locally deployed or a cloud provider managed database instance for the workload. 
+    database?:                  db.Database 
+
     ###### Other metadata info
     # Labels and annotations can be used to attach arbitrary metadata as key-value pairs to resources.
     labels?:                    {str:str}