feat(spanner): add connection property for enabling/disabling grpc-gcp

[olavloite]

Adds a connection property for enabling/disabling the grpc-gcp channel pool. This channel pool is the default, but can be disabled by setting this property to false. The connection will use Gax for channel pooling if grpc-gcp is disabled.

[gemini-code-assist]

Code Review

This pull request introduces the enableGrpcGcp connection property, allowing users to explicitly enable or disable the grpc-gcp extension. The implementation updates ConnectionOptions, ConnectionProperties, and SpannerPool to handle this new configuration, including logic to ensure Dynamic Channel Pooling is only active when grpc-gcp is not disabled. Feedback suggests improving documentation to clarify the interaction between enableDynamicChannelPool and enableGrpcGcp to prevent user confusion when both are set.