Implement backoff strategy

[tonykchen]

Issue #, if available: Closes #16

Description of changes:

• Moved common boto3 client logic to utils
• Added adaptive retry as documented here: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/retries.htm
• Added max_retry config to docs
• Added file headers to all .py files under src/
• Updated distribution name to agent-evaluation (pip install agent-evaluation)
• Added MANIFEST.in and long_description_content_type in setup.py to fix twine upload issues.

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