Add PYTHON_THREADPOOL_THREAD_COUNT app setting

[Hazhzeng]

Description

This is a dependency of enabling async thread pool. Getting this change into the codebase will allow us configure the threadpool more easily when benchmarking the async threadpool performance.

Fixes #675

---

Now we allow customer to configure number of thread in the synchronous threadpool by setting PYTHON_THREADPOOL_THREAD_COUNT application setting. If the application settings is not set, we will default it to 1.

I'm wondering whether we should call this PYTHON_SYNC_THREADPOOL_THREAD_COUNT.
If we introduce async thread pool later on, should we introduce a new PYTHON_ASYNC_THREADPOOL_THREAD_COUNT app settings? It is possible that if a function app mixes both sync and async functions, the actual number of threads will be 2x. Adding this new setting will has a better the transparency for customer to control their function app, but will make the app setting more complicate.

In my opinion, both async threadpool and sync threadpool should use the value in PYTHON_THREADPOOL_THREAD_COUNT.

PR information

• The title of the PR is clear and informative.
• There are a small number of commits, each of which has an informative message. This means that previously merged commits do not appear in the history of the PR. For information on cleaning up the commits in your pull request, see this page.
• If applicable, the PR references the bug/issue that it fixes in the description.
• New Unit tests were added for the changes made and CI is passing.

Quality of Code and Contribution Guidelines

• I have read the contribution guidelines.

[vrdmr]

@stefanushinardi - Could you please check if you want to change the name?

[stefanushinardi]

LGTM - putting my reasoning here for future reference: it makes sense to put the PYTHON_* prefix for this as without it users might get confused and use this config for other languages

[vrdmr]

LGTM.

some nits.