Expand ScalarMappable.set_array to accept array-like inputs

[aitikgupta]

PR Summary

imshow allows set_array to pass lists, Collection does not. (Since _ImageBase overrides set_array of ScalarMappable, adding the ability to pass array-like inputs, and copy the input so changing list after calling function doesn't affect the plots)
This PR expands the ScalarMappable class to make a copy of the original input and casting it to arrays.

Fixes #18841

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[aitikgupta]

Note: I didn't pay much attention to the test, as I think it would be better to change the ScalarMappable class itself, instead of overriding it multiple times.
A review is required before following up this approach.

[aitikgupta]

/cc possible reviewers
@anntzer @story645

[QuLogic]

This seems like something @efiring just looked at?

[jklymak]

as I think it would be better to change the ScalarMappable ...

If that fixes the problem more places, I'd suggest proposing that instead... As it is, this PR is a little scanty on justification.

[tacaswell]

Would it be better to fix this at the ScalarMappable level? 🐑 I should read more carefully before posting.

[aitikgupta]

I made the changes, but I'm not sure about the location of the test; there's no test_scalarmappable.py in the tests directory.
For now I've added the test in test_collections.py.

[aitikgupta]

I updated the PR summary, hopefully making this a bit easier to understand.

[efiring]

If #18480 goes in, set_array as modified by this PR will have to be further modified to restore the ability to set_array(None).

[jklymak]

I'm going to mark as draft until #18480 can be finished but please ping us if that takes too long!

[aitikgupta]

If #18480 goes in, set_array as modified by this PR will have to be further modified to restore the ability to set_array(None).

Rebased, finished the tests and handled this^

[aitikgupta]

@jklymak does this still require comment/discussion (label)?