chore: Register dynamic plugins and add feature checks

[benjreinhart]

SUMMARY

Following #14568, this PR removes conditional registration of the dynamic plugins resource and replaces it with a feature flag guard in a before_request hook.

cc @suddjian @robdiciuccio

[codecov]

Codecov Report

Merging #14650 (7a02e5a) into master (60f903f) will decrease coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #14650      +/-   ##
==========================================
- Coverage   77.54%   77.30%   -0.24%     
==========================================
  Files         958      958              
  Lines       48533    48541       +8     
  Branches     5702     5702              
==========================================
- Hits        37633    37525     -108     
- Misses      10699    10815     +116     
  Partials      201      201              

Flag	Coverage Δ
hive	?
mysql	81.37% <100.00%> (+<0.01%)	⬆️
postgres	81.40% <100.00%> (+<0.01%)	⬆️
presto	?
python	81.47% <100.00%> (-0.45%)	⬇️
sqlite	81.01% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/app.py	81.59% <100.00%> (+0.28%)	⬆️
superset/views/dynamic_plugins.py	100.00% <100.00%> (ø)
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	70.32% <0.00%> (-17.08%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-6.95%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	88.40% <0.00%> (-1.70%)	⬇️
superset/db_engine_specs/base.py	88.04% <0.00%> (-0.42%)	⬇️
superset/models/core.py	89.40% <0.00%> (-0.28%)	⬇️
superset/utils/core.py	88.87% <0.00%> (-0.13%)	⬇️

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