QBusiness IDC Integration

[nsusman]

Description of changes:

This pull request adds the Python lambda code to enable integration with the QBusiness Identity Center (IDC) as well as caching of credentials in DynamoDB.

The following additions were made:

• Added code to call create_token_with_iam to generate a session based on assuming an IAM role with permissions to QBusiness. This allows the application to integrate with IDC and generate temporary security credentials that can be reused to call the QBusiness chatsync operation.
• Added a reference to a new DynamoDB table to cache credentials generated via the IDC until their expiration time, including encryption and decryption of the credentials object using KMS. Because the passed JWT can only be used once unless refreshed, caching allows the plugin to re-use the generated credentials until their expiration

Please let me know if any part of these changes require further explanation or modification prior to merging. Can reference aws-samples/qnabot-on-aws-plugin-samples#29 in the plugin samples repo.

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