spanner: Add options for connecting to spanner omni

### Library Name @google-cloud/spanner ### A screenshot that you have tested with "Try this API". n/a ### What would you like to see in the library? Add explicit support for spanner omni with options similar to what's available on [Java](https://docs.cloud.google.com/spanner-omni/java). ### Describe alternatives you've considered Connection options can manually be set so as to connect to spanner omni following the lead in java's source code. However, an official support on how to connect to omni is preferred, as well as a proper vetting that the nodejs driver should work well with omni. ### Additional context/notes _No response_