[PJRT C API] Add a PJRT extension to register custom partitioner.
[PJRT C API] Add a PJRT extension to register custom partitioner.

- This extension has one C API which registers a custom partitioner with callbacks from the input.
- Update xla_client.register_custom_call_partitioner to take an optional PJRT_Api* input.
- Add xla_bridge.register_plugin_initialization_callbacks to register callbacks to be called with PJRT_Api* after plugins are discovered.
