fix: string broadcasting

[agoose77]

There are still some outstanding problems with string broadcasting, that pertain to parameter replacement in broadcast_and_apply.

The main cases are covered in the tests as xfail.

This PR fixes the broadcasting rules for parameters, with the following changes:

1. New list-types are not included in the parameter broadcasting, using a sentinel "NO_PARAMETERS" token
2. Record and union broadcasting considers only the parameters of the record/union layouts respectively; the non-record/non-union contents are nested in the records/unions that are build.

[agoose77]

This PR is done! We will possibly need to revisit this again in future; broadcasting is a fiddly thing, but it's definitely an improvement, and removes some xfails!

[jpivarski]

These are the right tests to make work. I had some comments on the implementation until I noticed that I was wrong, and deleted them.

It all looks good! Go ahead and merge when you're ready.

[codecov]

Codecov Report

Merging #2552 (83f9273) into main (228577b) will decrease coverage by 0.10%.
The diff coverage is 93.58%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/contents/indexedarray.py	78.99% <ø> (ø)
src/awkward/_broadcasting.py	93.02% <92.45%> (-0.58%)	⬇️
src/awkward/_parameters.py	82.22% <95.65%> (-0.74%)	⬇️
src/awkward/_connect/numpy.py	91.86% <100.00%> (ø)

... and 5 files with indirect coverage changes