[WV-2253] change header response to json

[mjacquot1]

NOTE: I also turned on the decorator for backup_one_table_to_s3_view in the interest of expediting functionality testing. This should not have any issue in prod due to all the try/except statements.

The previous issue is that NGINX would remove headers that contained a period .
This caused the code to run fine in local, but error in production.

After looking into RFC standards (https://datatracker.ietf.org/doc/html/rfc7230) the best course of action is to send JSON bodies over token headers.

This changes TokensManager functions to send and handle headers as JSON now.

Testing:
Tested functionality on local instance
Modified unit tests to handle JSON and expect string datetimes.

[DaleMcGrew]

👍 Thank you @mjacquot1!