feat: add pyinstrument integration to Flask API endpoints

[Nischay-Pro]

This PR provides profiling integration to developers for API endpoints, partially implementing the first half of #721.

To enable the profiler you need to export the FLEXMEASURES_PROFILE_REQUESTS environment variable with a value of True. Alternatively you can update the flexmeasures.cfg file reflecting the same.

The remaining steps are same as launching flexmeasures from the CLI.

Any profile reports generated would be saved in the profile_reports directory of the instance grouped into the date when the endpoint was profiled.

The file naming format for profile reports is pyinstrument_{ENDPOINT_NAME}

[nhoening]

Great work so far, and a good start (we might change how we store reports later, but we first should get started using it).

I have a few comments to make it safer to use.

Also, please add a changelog entry.

[nhoening]

Thanks for the update!

A small batch of docs improvements.

Can you hit "Resolve conversation" on all comments you've implemented?
If you disagree or didn't do it, then leave them open. Then I as a reviewer can easily see what's left.

[nhoening]

Looks good.

Please add an entry in documentation/changelog.rst for this infrastructure feature :)

[nhoening]

Great stuff, thanks!