docs: improve encrypted field adapter docs

[dpgaspar]

SUMMARY

Improved docs for SQLALCHEMY_ENCRYPTED_FIELD_TYPE_ADAPTER, provides an example on how to write a new adapter that changes the default encryption engine.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[geido]

LGTM!

[michael-s-molina]

LGTM

[codecov]

Codecov Report

Merging #21111 (90e76fe) into master (0c84203) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #21111   +/-   ##
=======================================
  Coverage   66.26%   66.26%           
=======================================
  Files        1770     1770           
  Lines       67543    67545    +2     
  Branches     7185     7185           
=======================================
+ Hits        44760    44762    +2     
  Misses      20944    20944           
  Partials     1839     1839           

Flag	Coverage Δ
hive	53.17% <ø> (+<0.01%)	⬆️
mysql	80.95% <ø> (+<0.01%)	⬆️
postgres	80.99% <ø> (+<0.01%)	⬆️
presto	53.06% <ø> (+<0.01%)	⬆️
python	81.48% <ø> (+<0.01%)	⬆️
sqlite	79.61% <ø> (+<0.01%)	⬆️
unit	50.73% <ø> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
superset/config.py	91.47% <ø> (ø)
superset/views/base.py	75.53% <0.00%> (+0.17%)	⬆️

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