Support for Building Layers

SAM CLI now supports building layers specified in template by specifying BuildMethod under a Metadata Attribute of the Layer Resource. If BuildMethod is not specified, the layer is not built.

Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: hello_world/
      Handler: app.lambda_handler
      Runtime: python3.8
      Layers:
        - !Ref MyLayer
        - !Ref MyLayer2
      Events:
        HelloWorld:
          Type: Api # More info about API Event Source: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#api
          Properties:
            Path: /hello
            Method: get
  MyLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      Description: Layer description
      ContentUri: 'my_layer/'
      CompatibleRuntimes:
        - python3.8
    Metadata:
      BuildMethod: python3.8
  MyLayer2:
    Type: AWS::Serverless::LayerVersion
    Properties:
      Description: Layer description
      ContentUri: 'my_layer2/'
      CompatibleRuntimes:
        - python3.8

sam build MyLayer build single layer.

sam build HelloWorldFunction builds function and the layer that the function uses.

23212ef - Build for Layers (#1936)
de8ad8e - Adding design doc for 'build for layers' (#1945)
24af60e - chore: version bump to 0.50.0 (#1998)
30b1016 - bug: security prompt - instrinsics in definition body (#1988)