add the ability to name the thread used by start_blocking_portal, and give it a default name

[graingert]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

I'd like to be able to call anyio.to_thread.start_blocking_portal(..., thread_name="example name") I'd also like a default name so I know what's going on in threading.enumerate()

Use case

I was porting a bespoke run-trio-in-thread code to BlockingPortal, the original code named their thread - but with a blocking portal I don't have that ability