[ENH] meta-object mixins

[fkiraly]

This PR adds meta-object mixins - previously, the package provided only classes inheriting from BaseObject or BaseEstimator, which can create multiple inheritance conflicts if downstream needs to inherit from an abstract desceendant of BaseObject or BaseEstimator.

This is also more in line with the actual use cases in sktime and skpro.

[codecov-commenter]

Codecov Report

Merging #216 (2f64a37) into main (d840e75) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
+ Coverage   83.59%   83.60%   +0.01%     
==========================================
  Files          43       43              
  Lines        2841     2843       +2     
==========================================
+ Hits         2375     2377       +2     
  Misses        466      466              

Files Changed	Coverage Δ
skbase/base/__init__.py	100.00% <100.00%> (ø)
skbase/base/_meta.py	36.16% <100.00%> (+0.57%)	⬆️