server: Begin accepting auth header and have it override callback URL values

[thomshutt]

What does this pull request do? Explain your changes. (required)

• Begin accepting a new Livepeer-Transcode-Configuration header in the HTTP Push segment endpoint.
• If present, this header should have the same format as the response from the "HTTP Auth Callback URL".
• If present and the Manifest ID from the path is not referring to an existing Manifest ID, then a call is still made to the callback URL as before (and fails if this call returns an error) but the fields it returns are overridden by the ones in the header
• If present and the Manifest ID in the URL already exists, this field is ignored

Specific updates (required)

• Create a new method for parsing the header and associated unit tests
• Add logic to override response from callback URL in the case outlined above. Unit test for this override behavior.

How did you test each of these updates (required)
✅ Wrote new unit tests
✅ TODO: Manual testing

Does this pull request close any open issues?

• Handle Livepeer-Transcode-Configuration header in HTTP push requests #2332

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[oscar-davids]

LGTM!

[cyberj0g]

Nitpick: is it ok to suppress error here?

[thomshutt]

Yeah, I can't see any actual situation where this wouldn't marshal but probably good practice to do something with the error anyway

[cyberj0g]

LGTM!