INTPYTHON-549 Rework the projection logic to allow reading list-of-struct data structures using a specified schema

[zpks]

see #284

[blink1073]

Thank you @zpks! Can you please add a test that covers your reported bug to test_arrow.py?

[blink1073]

It looks like this introduced some other failures. Are you able to run the test suite locally?

[zpks]

Hello! Thank you for looking, and my apologies for creating the PR prematurely

After some wrangling I was able to test locally.
The test for the desired feature works, it should display what's added.
I initially tried using the already-existing _create_nested_data() function with nested_elem=True, however that functionality does not work.

The change I made changes the projections from {"struct_col": {"field_1": True, "field_2": True}} to {"struct_col.field_1": True, "struct_col.field_2": True} - this works better with the mongo driver, as mongo interperts the dot as separating struct and field - and this also works for list-of-struct structures.
This meant I also had to edit the tests for the projections in order of those to run.
I have edited the tests for the projections - as those change, because in the previous format they errored out in the mongo read.

There was still an unrelated test failing TestExplicitPolarsApi.test_exceptions_for_unsupported_polar_types
I have not touched that code, so was unsure on how to fix those

[zpks]

interestingly enough my local ruff ran fine, but the github version finds a too-long line

[blink1073]

There was still an unrelated test failing TestExplicitPolarsApi.test_exceptions_for_unsupported_polar_types
I have not touched that code, so was unsure on how to fix those

If you merge from master that should be fixed by #288

[blink1073]

We have instructions for running the linters locally, or in this case you can copy-paste the affected lines from the diff.

https://github.com/mongodb-labs/mongo-arrow/blob/main/bindings/python/CONTRIBUTING.md#running-linters

[zpks]

I installed and initialized pre-commit, it ran all it's steps and passed - but I'll run it with tox too

[zpks]

Okay I rebased on main (I had to sync my fork first - again, my apology for the mess) and explicityly ran the linter, that fixed the issue - I suppose I had only installed pre-commit after the too-long line was committed. Thank you for your help!

[blink1073]

Thanks very much!