feat: Add "transfer_manager" module for concurrent uploads and downlo…

[rsaksham]

…ads as a preview feature (googleapis#844)

• checkpoint before design doc impl

• checkpoint

• more tests

• code and tests for transfer manager complete

• proactively close temp files when finished reading

• respond to comments; destroy tmp files as they are consumed

• Add system tests, docstrings, address feedback

• Respond to review comments

• verify md5 hash of downloaded file in test

• lint

• default empty strings for root arguments

• fix bug with blob constructor

• add warning about files not being deleted if their downloads fail

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• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕