feat: adding support for authentication type on UserAuthorizer

[BigTailWolf]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• [ ] 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
• [x] Ensure the tests and linter pass
• [x] Code coverage does not decrease (if any source code was changed)
• [x] Appropriate docs were updated (if necessary)

Fixes # ☕️

The change basically following the logic that NodeJS change: https://github.com/googleapis/google-auth-library-nodejs/pull/1814

The key point is telling the client how are the UserAuthorizer going to provide auth with token URI. Our current way is to have client_secret sending as part of the post url parameter. The STS endpoint won't allow that and they are not accepting client_secret field. Instead, the STS is using basic auth which takes a base64 encoding of client_id:client_secret.

Here the change is to provide a parameter to UserAuthorizer which auth from #RFC we are using and set the POST (Current way) as default. Then in the implementation, when sending the token request, we apply a basic auth header if the authentication type is set to BASIC.

If you write sample code, please follow the samples format.

[lsirac]

The library methods here return UserCredentials. Is this compatible with BYOID? We have ExternalAccountAuthorizedUserCredentials for BYOID.

[BigTailWolf]

The library methods here return UserCredentials. Is this compatible with BYOID? We have ExternalAccountAuthorizedUserCredentials for BYOID.

ExternalAccountAuthorizedUserCredentials won't need any changes to have the capability of BYOID. Cloud Code can choose this one if they want to switch. Per the sync with Cloud Code, they currently using UserAuthorizer for existing flow, what we do is just adding the capability to UserAuthorizer calling token endpoint with basic auth header.

[sonarcloud[bot]]

Quality Gate passed

Issues
3 New issues
0 Accepted issues

Measures
0 Security Hotspots
92.6% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud