Support setting principal and SASL extensions in oauth_cb, handle failures

[Manicben]

Fixes #1394

Extends the return values of the oauth_cb to include a principal string, and a dict of SASL extensions, which must be strings matching [A-Za-z]+. These return values are optional, meaning that existing callbacks still work.

When an exception is raised in the callback, rd_kafka_oauthbearer_set_token_failure is called to ensure another refresh is scheduled 10s later. This is for the case where the callback raising an exception is due to a token not being able to be fetched, which may succeed later. Without this, raising exceptions in the callback result in the client never re-fetching a token unless fully restarted.

Suggestions regarding the latter part are appreciated, an alternative would be to allow a return value of None to signal token fetch failure and the need to retry later. However raising an exception is a bit more Pythonic, so maybe raising a certain exception type should be the way to signal a failure.

[CLAassistant]

All committers have signed the CLA.

[mhowlett]

thanks for the PR @Manicben - we'll try to get to this soon.

[emasab]

Thanks @Manicben . I'm reviewing it today

[emasab]

LGTM, I've just added a commit to remove the global variables