Add serializers middleware logic for protocol generator awsjson10.

[nathanhit]

Issue #, if available:

Description of changes:

• Update Protocol Generator request serializer generation to a cleaner and complete build.
• Add a new class SerializeMiddleware that handles all generation for request serializers.
• Add support for shared serializers to handle complex input shapes, nested within serializer request generaiton.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.