[AIRFLOW-7084] Lazy initialize plugins for each entrypoint

[mik-laj]

When we have custom executors and operators loaded using the operators property in plugins to avoid loading classes in the scheduler, operators are still loaded. After this change, the properties will be read only when needed.

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Issue link: AIRFLOW-7084

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[codecov-io]

Codecov Report

Merging #7758 into master will decrease coverage by 0.05%.
The diff coverage is 88.17%.

@@            Coverage Diff             @@
##           master    #7758      +/-   ##
==========================================
- Coverage   86.98%   86.93%   -0.06%     
==========================================
  Files         915      920       +5     
  Lines       44228    44499     +271     
==========================================
+ Hits        38473    38685     +212     
- Misses       5755     5814      +59     

Impacted Files	Coverage Δ
airflow/serialization/serialized_objects.py	90.09% <66.66%> (-0.52%)	⬇️
airflow/models/baseoperator.py	95.68% <80.00%> (-0.37%)	⬇️
airflow/plugins_manager.py	88.58% <90.78%> (-1.56%)	⬇️
airflow/www/app.py	94.28% <100.00%> (ø)
...le_dags/example_passing_params_via_test_command.py	100.00% <0.00%> (ø)
...rflow/providers/google/firebase/hooks/firestore.py	97.29% <0.00%> (ø)
.../google/firebase/example_dags/example_firestore.py	100.00% <0.00%> (ø)
.../providers/google/cloud/operators/presto_to_gcs.py	38.66% <0.00%> (ø)
...google/cloud/example_dags/example_presto_to_gcs.py	100.00% <0.00%> (ø)
...w/providers/google/firebase/operators/firestore.py	95.65% <0.00%> (ø)
... and 4 more

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