add flags for set worker thread name

[yanglimingcn]

What problem does this PR solve?

Issue Number: None

Problem Summary:

In some scenarios, services may prefer to keep worker thread names unchanged, as external monitoring and alerting systems already rely on custom-defined thread names. To accommodate this, we've introduced a gflag to control whether bRPC should modify worker thread names.

When thread name modification is disabled through this configuration, users can alternatively set worker thread names within the run_worker_startfn function.

What is changed and the side effects?

Changed:

Side effects:

• Performance effects:

• Breaking backward compatibility:

---

Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wwbmmm]

LGTM

[chenBright]

LGTM