[AIRFLOW-5947] Make the json backend pluggable for DAG Serialization

[kaxil]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-5947

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Allow users the option to choose the JSON library of their choice for DAG Serialization.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

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  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

[codecov-io]

Codecov Report

Merging #6630 into master will decrease coverage by 0.3%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6630      +/-   ##
==========================================
- Coverage   83.79%   83.48%   -0.31%     
==========================================
  Files         669      669              
  Lines       37548    37552       +4     
==========================================
- Hits        31464    31351     -113     
- Misses       6084     6201     +117

Impacted Files	Coverage Δ
airflow/serialization/serialized_dag.py	96% <ø> (ø)	⬆️
airflow/settings.py	88.96% <100%> (+0.15%)	⬆️
airflow/serialization/serialization.py	82.57% <100%> (ø)	⬆️
airflow/models/serialized_dag.py	85.71% <100%> (+0.34%)	⬆️
airflow/serialization/json_schema.py	81.81% <100%> (ø)	⬆️
airflow/operators/mysql_operator.py	100% <0%> (ø)	⬆️
airflow/operators/mysql_to_hive.py	100% <0%> (ø)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
... and 9 more

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[potiuk]

Nice.