Support Stimulus ID's in Scheduler with ContextVars

[sjperkins]

• Closes Transition tracing for scheduler task transitions #5849
• Tests added / passed
• Passes pre-commit run --all-files

[github-actions]

Unit Test Results

       18 files  ±0         18 suites  ±0   9h 27m 19s ⏱️ + 32m 12s
  2 725 tests +1    2 643 ✔️ +  2       81 💤 ±0  1 ❌  - 1 
24 373 runs  +9  23 159 ✔️ +14  1 213 💤  - 4  1 ❌  - 1 

For more details on these failures, see this check.

Results for commit d0aa434. ± Comparison against base commit bddc3f2.

♻️ This comment has been updated with latest results.

[sjperkins]

.take

[fjetter]

Some things appeared to be a bit more complex than I anticipated and I became a bit nervous of whether contextvars are the right approach. I think there should only be one global thing (I might be wrong) and I went on implementing something on top of this PR, see #6068

We should sync up and align what the next steps are

[crusaderky]

I'm a bit swarmed right now and can't keep up with the review process - I'm leaving it to @fjetter

[sjperkins]

Some things appeared to be a bit more complex than I anticipated and I became a bit nervous of whether contextvars are the right approach. I think there should only be one global thing (I might be wrong) and I went on implementing something on top of this PR, see #6068

We should sync up and align what the next steps are

I've changed the ContextVar to a single global variable, rather than a per Scheduler instance ContextVar. I don't think this changes the underlying functionality much, as the ContextVar is only used internally in the Scheduler at present. I think it would be great to to do a follow up PR that extends this to Client and Workers.

[sjperkins]

Introduces too much indentation into the diff. Closing in favour of #6083