Support for `client_credentials` grant types

[euank]

Specifically RFC 6749, section 2.3.

Is implementing that section of the RFC in-scope for this library?
Is there a reason it isn't currently implemented, or a reason not to implement it?

[causal-agent]

Isn't section 2.3.1 implemented here? https://github.com/programble/inth-oauth2/blob/master/src/client/mod.rs#L115-L125

[euank]

Apologies, I linked to the wrong thing! It should have been 4.4.2, where the grant_type can be client_credentials to indicate a client credential flow vs user auth flow.

From grepping for client_credentials and reading the docs, there doesn't appear to be a way to do that flow.

[causal-agent]

There's no reason this library couldn't implement it, I just haven't run into the need for it.