[core][1/N] Make executor to be a long-running Python thread

[kevin85421]

Why are these changes needed?

This is the same as #50644

Currently, tasks are sometimes executed on the main thread and sometimes on other threads which is created by C++. However, C++ threads are only considered Python threads when they execute Python functions. Hence, when a task finishes, the thread-local state will be garbage-collected. See #46336 (comment) for more details.

This PR uses the CPython API to treat C++ threads as Python threads, even if they do not execute Python functions.
https://docs.python.org/3/c-api/init.html#non-python-created-threads

This PR handles synchronous tasks. After this PR is merged, the follow-up tasks are:

• Always create a default executor.
• Move the constructor to the default executor.
• Support asynchronous tasks.

Related issue number

Part of #46336

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(