feat: Introduce asynchronous DDL execution modes

[bhatt4982]

• support modes SYNC, ASYNC and ASYNC_WAIT
• returns operationIDs for DDL statements.

[gemini-code-assist]

Summary of Changes

Hello @bhatt4982, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the flexibility of DDL (Data Definition Language) execution by introducing explicit asynchronous modes. Users can now choose to execute DDL statements synchronously (waiting for completion), fully asynchronously (returning immediately with an operation ID), or in a hybrid asynchronous-wait mode (waiting for a fixed duration before returning an operation ID if not completed). This change provides greater control over DDL operations, enabling non-blocking schema changes and better integration with applications that require immediate feedback or background processing.

Highlights

• New DDL Execution Modes: Introduced SYNC, ASYNC, and ASYNC_WAIT modes for DDL statements, configurable via a new connection property.
• Operation ID Retrieval: DDL statements executed in ASYNC or ASYNC_WAIT modes now return an OperationID through the SpannerResult interface.
• Asynchronous Batch DDL: RUN BATCH DDL operations can now return their OperationID when executed asynchronously.

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[gemini-code-assist]

Code Review

This pull request introduces asynchronous DDL execution modes (SYNC, ASYNC, and ASYNC_WAIT) and provides operation IDs for DDL statements. The implementation correctly handles the different execution behaviors for each mode, including the timeout logic for ASYNC_WAIT. New test cases have been added to validate these modes, and the mocking infrastructure has been updated to support long-running operations. The changes are well-structured and align with the stated objectives of the pull request.

[gemini-code-assist]

Modifying a global variable (ddlAsyncWaitTimeout) directly within a test, even with a defer to reset it, can introduce subtle race conditions or non-deterministic behavior if tests are run concurrently and the reset is not guaranteed to happen before another test reads the value. Consider making this timeout configurable per test or using a test-specific mock to ensure better isolation and parallel test execution safety.

[gemini-code-assist]

The ddlAsyncWaitTimeout is defined as a global variable. While its purpose is clear, global mutable state can be a source of unexpected side effects and make testing or concurrent usage more complex. Consider making this timeout configurable through connection properties or passing it as a parameter to waitForDDLOperation to improve modularity and reduce potential coupling issues.

[olavloite]

This should indeed be a connection property. 10s sounds like a reasonable default, but it should be something that an application developer can change to fit their needs.