Add Bedrock Knowledge Base datasource support

[jasonbrd]

This adds backend components to support Bedrock Knowledge Bases in Amplify Assistants.

• Adds handler in create assistant to support Bedrock KB, including validation the KB exists upon creation or editing of assistant
• Is feature flagged
• Merges response from Bedrock Retrieve API into the Amplify RAG pipeline, allowing for combination of Bedrock KB + assistant documents