Update IChatClient with support from latest bedrock runtime / M.E.AI

[stephentoub]

Description

Update IChatClient with support from latest bedrock runtime / M.E.AI

Motivation and Context

• Adds support for multi-modal tool returns.
• Adds support for citations with URIs.
• Adds a ton of tests verifying IChatClient behavior around the underlying IAmazonBedrockRuntime.

Testing

The PR includes a lot of tests for the IChatClient implementation, which previously had few-to-none.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the README document
• I have added tests to cover my changes
• All new and existing tests passed

License

• I confirm that this pull request can be released under the Apache 2 license