chore: deprecate experimentalHost and add instanceType support for Spanner Omni

[sagnghos]

This PR deprecates the ExperimentalHost option/flag and introduces the InstanceType configuration parameter supporting CLOUD and OMNI instances. This aligns the repository with modern Spanner architecture design patterns and improves backward-compatible fallback logic.

Key Changes

1. Deprecated ExperimentalHost:
   • Marked ExperimentalHost as deprecated in both the client Options (options.go) and wrapper Options (spanner.go).
   • Warns users when using the deprecated --experimentalHost flag in the CLI launcher.
   • Automatically fallback-maps ExperimentalHost = true to InstanceType = OMNI and sets plain-text insecure transport credentials to preserve backward compatibility.
2. Introduced InstanceType:
   • Added a new InstanceType string enum field with CLOUD (default) and OMNI options.
   • Renamed internal helper functions (createExperimentalHostCredentials -> createSpannerOmniCredentials) to reflect general Spanner Omni connectivity.
3. Database URI Normalization:
   • Automatically prefixes simple database names with "projects/default/instances/default/databases/" when InstanceType == OMNI (or the deprecated ExperimentalHost == true).
4. Enhanced Test Coverage:
   • Split existing test suites in both client options and wrapper clusters to test deprecated fallback behaviors and the new explicit InstanceType parameter independently.

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How to Run/Test Against Spanner Omni

Running Integration Tests

To execute integration tests against a locally running Spanner Omni / emulator instance:

SPANNER_INSTANCE_TYPE=OMNI go test -v ./cassandra/gocql -tags=integration -args -target=spanner -spanner-endpoint=<omni-host>:<port>

Example (running against an instance at port 15000):

SPANNER_INSTANCE_TYPE=OMNI go test -v ./cassandra/gocql -tags=integration -args -target=spanner -spanner-endpoint=localhost:15000

Running the CLI Launcher

To launch the Cassandra-to-Spanner adapter proxy using the new instanceType parameter:

go run cassandra_launcher.go -db test-db -endpoint <omni-host>:<port> -instanceType OMNI -usePlainText -tcp ":9042" -grpc-channels 1

Example:

go run cassandra_launcher.go -db test-db -endpoint localhost:15000 -instanceType OMNI -usePlainText -tcp ":9042" -grpc-channels 1

Similar PRs for reference:

googleapis/google-cloud-go#14694
googleapis/java-spanner-cassandra#357