[pjrt][xla:ffi] Add XLA FFI execution context to PjRt ExecuteContext
[pjrt][xla:ffi] Add XLA FFI execution context to PjRt ExecuteContext

- Extend PjRt ExecuteContext base class to always include XLA FFI execution context so that end-users can pass data to FFI handlers.

- Add an end-to-end test to SE GPU PjrtClient.

PJRT C API changes will be in the followup PRs.
