Make FlaskApp number of threads configurable

[nmoreaud]

Hello

We are migrating a Connexion v2 application to Connexion v3.
We are trying to use FlaskApp, however we encounter bugs in our services due to the number "worker threads" (10 WSGI threads) which is no longer configurable.
I guess the long term solution will be to drop Flask dependency in our software, but for now we'd like to make it work with the backward compatibility layer.
Can you please make the number of worker threads configurable?

https://github.com/spec-first/connexion/blob/main/connexion/apps/flask.py#L133
https://github.com/abersheeran/a2wsgi/blob/master/a2wsgi/wsgi.py#L157
https://connexion.readthedocs.io/en/latest/v3.html#running-the-application

Additional info:

Output of the commands:

Python 3.11.10
connexion==3.1.0

[nmoreaud]

Do you plan to do this? Otherwise I'll have to monkey patch it.

[RobbeSneyders]

I would welcome a PR with this change.

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