support client credentials grant from `Connection.authenticate_oidc()`

[soxofaan]

Connection.authenticate_oidc() currently automatically switches between refresh token auth and device code flow auth depending on the presence of recent refresh tokens.

With growing interest in client credential auth, we should also add support for that under authenticate_oidc, that is triggered from a config setting, or the presence of certain environment variables.

That will make it possible to use a generic connection.authenticate_oidc() snippet in code samples, notebooks, ..., which will then automatically use an appropriate auth method (device code for first time user, client credentials for CI/CD test use cases, ...)

[soxofaan]

(internal refs: CDAS-619, CDAS-622)

[soxofaan]

worked on PR last week: #429

[soxofaan]

already merged #429 to have the basics in master

further finetuning:

• docs explaining this feature better (e.g. document full feature set of authenticate_oidc
• better unification of config and envvar handling

[soxofaan]

improved documentation now as well

time to close