[ENH] tag manager mixin

[fkiraly]

port of sktime/sktime#3630 to skbase

This PR factors out the tag management logic into a _FlagManager mixin, and generalizes the functionality so it can be used for multiple fields of tags.

Advantages:

• increases cohesion and reduces coupling, separates concerns in the BaseObject class: state/parameter management and tag management
• prepares the ground for managing configuration fields, e.g., for implementing a set_config similar to the new sklearn feature

Test coverage is already there for the base case, but should be added for a different tag field. Will do that with the PR that adds config handling.

(I called the flag manager since not all flags are tags, and interactions with the sklearn _get_tags)

A STEP is available here: sktime/enhancement-proposals#29

[codecov-commenter]

Codecov Report

Merging #138 (db9846d) into main (fa53055) will increase coverage by 0.20%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
+ Coverage   82.43%   82.64%   +0.20%     
==========================================
  Files          31       32       +1     
  Lines        2289     2316      +27     
==========================================
+ Hits         1887     1914      +27     
  Misses        402      402              

Impacted Files	Coverage Δ
skbase/base/_base.py	81.81% <100.00%> (-1.40%)	⬇️
skbase/base/_tagmanager.py	100.00% <100.00%> (ø)

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[RNKuhns]

@fkiraly see the requested docstring requests on #140 (which appears to have some logic repeated there -- likely since you needed some of this logic to add config interface).

[fkiraly]

Addressed, see replies in #140

[RNKuhns]

LGTM now.