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.