chore(spanner): deprecate experimental host option/parameter to replace with Spanner Omni

[sagnghos]

This PR deprecates the experimentalHost / isExperimentalHost option and property across the Cloud Spanner client libraries (SpannerOptions and ConnectionOptions) and introduces a unified type option and connection parameter (supporting the values cloud, omni, and emulator).

To connect to a Spanner Omni instance, users should now use the setType(InstanceType.OMNI) builder option or pass the connection string query parameter ?type=omni.

Refer to discussion: Spanner Client Library Configuration for Omni

Currently, setting the type to cloud or emulator acts as a no-op (since the client library automatically resolves default behaviors for standard cloud and emulator setups). However, introducing this option makes the client library highly extendable, enabling us to enforce or optimize custom behaviors tailored to specific Spanner instance types in the future.

[gemini-code-assist]

Code Review

This pull request rebrands 'experimental host' features as 'Spanner Omni' and introduces a formal InstanceType enum (CLOUD, OMNI, EMULATOR) to manage connection types. The changes include updating configuration constants, renaming internal methods and helper classes, and deprecating older 'experimental host' properties in favor of the new type property. Feedback focuses on the setType(InstanceType.OMNI) implementation, specifically regarding a strict order dependency where setHost must be called before setType, which violates standard builder patterns. Additionally, the current implementation has destructive side effects that can silently overwrite user-provided configurations for project IDs and session pool options, especially when re-invoked through methods like usePlainText().