fix(spanner): ensure executeQueryAsync is non-blocking

[olavloite]

When executeQueryAsync(..) was called for a read-only transaction, the method would execute a blocking BeginTransaction RPC for the first query. This change ensures that the BeginTransaction RPC instead is executed using the background executor thread pool.

[gemini-code-assist]

Code Review

This pull request implements lazy initialization for asynchronous result sets by wrapping internal read and query operations in a memoized supplier. This change ensures that methods like readAsync and executeQueryAsync are non-blocking, deferring the actual execution until the result set is accessed. New tests verify this behavior and ensure that transactions are only initiated once. A review comment suggests a minor refactor in the test code to handle result set state transitions more efficiently by avoiding redundant calls to tryNext().