feat: config to customize bootstrap data overrides

[suddjian]

SUMMARY

Motivation:

• Preset needs to add some custom sequential color palettes for specific users
• color palettes are delivered in bootstrap data, via the extra_sequential_color_schemes and extra_categorical_color_schemes fields.
• Wanted to allow general changes to common bootstrap data as well, to allow open-ended support for customizations in Superset deployments.
• Decided to add a function which can be configured to get these bootstrap_data.common overrides.

TESTING INSTRUCTIONS

1. Set EXTRA_SEQUENTIAL_COLOR_SCHEMES to a function that returns a dict of { "foo": "bar" }
2. Verify that Superset Frontend still renders, and that { "foo": "bar" } was delivered in the bootstrap data.

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

[codecov]

Codecov Report

Merging #16386 (1ee961d) into master (a547dcb) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #16386      +/-   ##
==========================================
+ Coverage   76.63%   76.68%   +0.04%     
==========================================
  Files        1000     1000              
  Lines       53424    53980     +556     
  Branches     6818     6818              
==========================================
+ Hits        40943    41392     +449     
- Misses      12245    12352     +107     
  Partials      236      236              

Flag	Coverage Δ
hive	81.38% <100.00%> (+0.04%)	⬆️
javascript	70.76% <ø> (ø)
mysql	81.72% <100.00%> (+0.12%)	⬆️
postgres	81.78% <100.00%> (+0.12%)	⬆️
presto	?
python	82.14% <100.00%> (-0.03%)	⬇️
sqlite	81.43% <100.00%> (+0.13%)	⬆️

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

Impacted Files	Coverage Δ
superset/config.py	91.33% <100.00%> (+0.02%)	⬆️
superset/views/base.py	76.02% <100.00%> (+0.16%)	⬆️
superset/charts/post_processing.py	82.92% <0.00%> (-11.25%)	⬇️
superset/db_engine_specs/presto.py	84.51% <0.00%> (-5.86%)	⬇️
superset/examples/deck.py	10.25% <0.00%> (-2.25%)	⬇️
superset/utils/csv.py	98.21% <0.00%> (-1.79%)	⬇️
superset/connectors/sqla/models.py	88.28% <0.00%> (-1.43%)	⬇️
superset/reports/commands/execute.py	92.03% <0.00%> (-0.57%)	⬇️
superset/models/core.py	89.14% <0.00%> (-0.26%)	⬇️
superset/reports/notifications/email.py	100.00% <0.00%> (ø)
... and 7 more

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[amitmiran137]

Lgtm

[michael-s-molina]

LGTM