Add Hawk Authentication for sources

[VictorD-Veolia]

Integration of HAWK authentication for HTTP sources

[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[VictorD-Veolia]

@googlebot I signed it!

[bdmogal]

@VictorD-Veolia thanks for the PR. Could you share a short document describing the use case for this, since I'm not familiar with HAWK?

[VictorD-Veolia]

Hi @bdmogal, HAWK is an HTTP authentication scheme providing mechanisms for making authenticated HTTP requests with partial cryptographic verification of the request and response, covering the HTTP method, request URI, host, and optionally the request payload.

Some teams may prefer HAWK authentication over Oauth2 for their APIs as a more secure option when additional verification must be done on requests and responses.

[avirmani]

@VictorD-Veolia - is there a small 1-pager that you can share to explain this a bit more, pls? Also, we should look at adding some unit tests here?