Rename MetadataEndpoint._grant_types -> _grant_types_supported

[bennr01]

Hello,

this PR renames the _grant_types attribute of the MetadataEndpoint to _grant_types_supported.
Initially introduced in 6bd865b, the _grant_types attribute (type list) collides with the _grant_types attribute (type dict) of the TokenEndpoint when inheriting from both classes.

This caused a problem: If I want to create an Server-like combined endpoint inheriting from both the Server and MetadataEndpoint classes and pass a reference to an instance of this class to the constructor of the MetadataEndpoint, initializing the MetadataEndpoint will fail due to it setting self._grant_types to a list but expecting each of the endpoint working on (which, in my case, includes the metadata endpoint itself) to be of type dict.

For example, the following code would fail:

from oauthlib import oauth2


class ServerWithMetadata(oauth2.Server, oauth2.MetadataEndpoint):
    """
    An extension of L{oauth2.Server} featuring the L{oauth2.MetaDataEndpoint}.
    """
    def __init__(
        self,
        request_validator,
        token_expires_in=None,
        token_generator=None,
        refresh_token_generator=None,
        claims={},
        *args,
        **kwargs,
    ):
        oauth2.Server.__init__(
            self,
            request_validator,
            token_expires_in=token_expires_in,
            token_generator=token_generator,
            refresh_token_generator=refresh_token_generator,
            *args,
            **kwargs,
        )
        oauth2.MetadataEndpoint.__init__(
            self,
            [self],
            claims=claims,
            raise_errors=True,
        )

A possible solution is renaming one of the _grant_types attributes. This PR does this.

[thedrow]

Can we write a test to ensure the bug doesn't regress?

[bennr01]

@thedrow I've added a regression test for the issue. The new test passes with the fix but fails without it. I wasn't exactly sure where to implement it, but as the changes were solely within the code of the MetadataEndpoint I've decided to put the test in the corresponding file. Is this ok?

[bennr01]

Hi, are there any problems with the changes in this pull request? While one of the test build fails, this is only caused by bandit complaining over the user of possible hardcoded tokens (e,g, Bearer and access_token), which are failures unrelated to this PR and mostly false positives.