[ENH] improved deep_equals utility - plugins for custom types

[fkiraly]

This PR improves the deep_equals utility by introducing the ability to provide custom plugins.

Ultimately, this enables more flexibility in soft dependencies for data containers such as polars (see, e.g., sktime/sktime#5423) throughout the sktime package ecosystem, as deep_equals is used for extrinsic testing and validity checking.
Similarly, the ForecastingHorizon check should be moved as a plugin into sktime.

In addition, makes the following changes:

• splits up the deep_equals file into a module with multiple files

Depends on #239.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (d55abc6) 83.72% compared to head (04b048d) 84.18%.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #238      +/-   ##
==========================================
+ Coverage   83.72%   84.18%   +0.45%     
==========================================
  Files          41       43       +2     
  Lines        2857     2889      +32     
==========================================
+ Hits         2392     2432      +40     
+ Misses        465      457       -8     

Files	Coverage Δ
skbase/utils/deep_equals/__init__.py	100.00% <100.00%> (ø)
skbase/utils/tests/test_deep_equals.py	100.00% <100.00%> (ø)
skbase/utils/deep_equals/_common.py	92.30% <92.30%> (ø)
skbase/utils/deep_equals/_deep_equals.py	74.19% <84.12%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.