Support for dynamic declaration storage

[jessepeterson]

Adds support for dynamic declarations via interfaces and adapters over declaration storage.

The three declaration interface methods, RetrieveTokensJSON(), RetrieveDeclarationItemsJSON(), and RetrieveEnrollmentDeclarationJSON() are the primary ways by which declarations, declaration items, and tokens are served to clients. We introduce a NewJSONAdapt() service which can dynamically supply declaration items and tokens using a new storage interface method, RetrieveDeclarationItems().

This new interface opens the possibility of composing multiple sets of declarations into a single declaration items. And we do just that with the NewMulti() service which composes multiple declaration storages using "fall-through" to provide a unified "view" of declarations to an enrollment.

Included is an implementation of a "shard" declaration storage. This is combined with the normal backend storage (using the -shard switch to the server) to be simultaneously served with the persisted declarations from disk or database. From the docs:

[shard declaration storage is] an always-on management properties declaration for every enrollment. It contains a shard payload key which is a dynamically computed integer between 0 and 100, inclusive, based on the enrollment ID. This shard key can then be used in activation declaration predicates. For example (@property(shard) <= 75). The identifier of this dynamic declaration is com.github.jessepeterson.kmfddm.storage.shard.v1; the Server Token includes the shard number. It is "static" in that it should not change for any given enrollment.

Also some other minor changes:

• Move builder code into separate package from core ddm package
• Unify declaration "not found" return types (and properly support 404 in return of declaration)
• Introduce sqlc to MySQL backend (though, not fully convert over yet)
• Changes, updates, additions to tests
• Separate DDM storage interfaces and improve docs

[jessepeterson]

As an example, here is what the declarations items looks using the shard backend (and no other declarations):

$ ./tools/ddm-declaration-items.sh 'abc' | jq .
{
  "Declarations": {
    "Activations": [],
    "Assets": [],
    "Configurations": [],
    "Management": [
      {
        "Identifier": "com.github.jessepeterson.kmfddm.storage.shard.v1",
        "ServerToken": "shard=11;version=1"
      }
    ]
  },
  "DeclarationsToken": "89ad6b6991ef675e"

And here is what the actual declaration looks like:

$ ./tools/ddm-declaration.sh 'abc' 'management/com.github.jessepeterson.kmfddm.storage.shard.v1'
{
	"Type": "com.apple.management.properties",
	"Identifier": "com.github.jessepeterson.kmfddm.storage.shard.v1",
	"Payload": {
		"shard": 11
	},
	"ServerToken": "shard=11;version=1"
}