Add the ability to mock BatchGet and MultiTableBatchGet operations

[96malhar]

Description

This PR adds the ability to mock BatchGet and MultiTableBatchGet operations in DynamoDB.

• The IDynamoDBContext.CreateBatchGet<T>(...) method now returns a generic IBatchGet<T> interface
• The IDynamoDBContext.CreateMultiTableBatchGet(...) method now returns a IMultiTableBatchGet interface.

The IBatchGet<T> interface also implements the non generic IBatchGet which defines the Execute and ExecuteAsync method to carry out the BatchGet operations.

I have added new unit tests to showcase how a user will mock these interfaces.

Motivation and Context

DOTNET-7676

Testing

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