[FFI] Allow introspecting the set of registered custom calls.
[FFI] Allow introspecting the set of registered custom calls.

The custom call API only has a lookup operation today, not an iteration operation.
Having an iteration option allows inspecting the entire set of enabled calls to, say,
improve error messages or filter provided HLOs to only those that support expected
custom calls.

Both custom call API versions that are currently supported by xla_client are supported.
The proto defines more, but since they aren't actually register-able there isn't anything
we can do yet.
