[XLA] Extract BackendConfig-related functionality out of `hlo_instruction.h/cc`.
[XLA] Extract BackendConfig-related functionality out of `hlo_instruction.h/cc`.
- `BackendConfigRep` is now in `backend_config.h/cc` and renamed to `BackendConfigWrapper`
- `BackendConfigToRawString` is also moved to `backend_config.h/cc`
