[AIRFLOW-4824] Add charset handling for SqlAlchemy engine for MySqlHook

[zuku1985]

Airflow should handle various charsets for connections with MySQL dbs.
This change allows to set charset in extra field of a connection when
using SqlAlchemy engine.

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-4824
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
  • In case you are proposing a fundamental code change, you need to create an Airflow Improvement Proposal (AIP).
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Description

• Here are some details about my PR, including screenshots of any UI changes:
  Airflow should handle various charsets for connections with MySQL dbs.
  This change allows to set charset in extra field of a connection when
  using SqlAlchemy engine.

Tests

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

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  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 #6816 into master will decrease coverage by 0.13%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #6816      +/-   ##
==========================================
- Coverage   84.45%   84.31%   -0.14%     
==========================================
  Files         676      676              
  Lines       38310    38317       +7     
==========================================
- Hits        32354    32307      -47     
- Misses       5956     6010      +54

Impacted Files	Coverage Δ
airflow/hooks/mysql_hook.py	92.77% <85.71%> (-0.65%)	⬇️
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%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0%> (-23.53%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	78.2% <0%> (-20.52%)	⬇️
airflow/hooks/hive_hooks.py	77.6% <0%> (ø)	⬆️
airflow/models/connection.py	68.96% <0%> (ø)	⬆️
... and 5 more

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[potiuk]

Thanks @zuku1985 - yet another result of the workshop! :)