client.register() has no easy way to pass in an Initial Access Token

[schlenk]

If a OP has dynamic client registration open, but requires an Initial Access Token to access the registration endpoint, this is currently not possible via client.register().

This can happen if the OP follows the spec:
https://openid.net/specs/openid-connect-registration-1_0.html#ClientRegistration

The OpenID Provider MAY require an Initial Access Token that is provisioned out-of-band (in a manner that is out of scope for this specification) to restrict registration requests to only authorized Clients or developers.

One can hack around it with a custom requests auth class:

client.request_args['auth'] = HTTPBearerAuth(registration_token)
registration_info = client.register(registration_endpoint, **args)
del client.request_args['auth']

This feels a bit dirty. The register() API should take some auth information via a kwarg directly.