sphinx_ext: New directive for supported methods

Until now many resources, but not all and not consistently list their
supported methods (e.g. “Supports the following commands: ``GET`` …”).
Not only is it easy for this list to get out of date, but it would also
be nice if it listed the required access permissions.

This patch adds a new directive to insert a table listing all supported
methods and their access permissions for a resource.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>