Implemented Rain::Env for pkg command

[null93]

Description of Changes:

I find it very useful to inject environmental variables into the template. Some use cases would include injecting a default AMI name into a template based on what environment the template is being packaged for (staging/production). For me this happens during CI/CD.

Expected Behavior

The name of the environmental variable is passed as the only argument to Rain::Env.

If the environmental variable does not exist, then an error is returned, otherwise the value of the environmental variable is injected into the template as a string.

Example

Given a file named template.yaml with the following contents:

AWSTemplateFormatVersion: "2010-09-09"

Description: Template generated by rain

Parameters:
  ImageId:
    Type: String
    Default: !Rain::Env AMI_IMAGE

Resources:
  MyInstance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: !Ref ImageId
      InstanceType: c6g.xlarge
      # ...

Running the following command:

export AMI_IMAGE=ami-1234567890
rain pkg template.yml

Should result in a template that looks like this:

AWSTemplateFormatVersion: "2010-09-09"

Description: Template generated by rain

Parameters:
  ImageId:
    Type: String
    Default: ami-1234567890

Resources:
  MyInstance:
    Type: AWS::EC2::Instance
    Properties:
      ImageId: !Ref ImageId
      InstanceType: c6g.xlarge
      # ...

Additional Changes

I added some tests to for #136 as promised :)

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