class WireSyncBufferClientImpl

|WireSyncBufferClientImpl| implements synchronous FIDL calls with

caller-provided buffers. It contains an unowned transport handle.

TODO(https://fxbug.dev/42159083): Consider merging this implementation with

|WireWeakSyncBufferClientImpl| to support thread-safe teardown of

|fidl::WireSyncClient|s.