chore: rename "uiOverrideRegistry" -> "extensionsRegistry"

[suddjian]

SUMMARY

Renaming uiOverrideRegistry to the better name extensionsRegistry.

Kept the old name around for backwards compatibility. It should be removed later.

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

[codecov]

Codecov Report

Merging #20628 (ca883ad) into master (94b3d2f) will decrease coverage by 0.16%.
The diff coverage is 83.56%.

❗ Current head ca883ad differs from pull request most recent head 9b924b8. Consider uploading reports for the commit 9b924b8 to get more accurate results

@@            Coverage Diff             @@
##           master   #20628      +/-   ##
==========================================
- Coverage   66.78%   66.62%   -0.17%     
==========================================
  Files        1754     1751       -3     
  Lines       65592    65581      -11     
  Branches     6938     6938              
==========================================
- Hits        43808    43695     -113     
- Misses      20026    20126     +100     
- Partials     1758     1760       +2     

Flag	Coverage Δ
hive	?
javascript	51.85% <53.84%> (+0.03%)	⬆️
presto	?
sqlite	?

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

Impacted Files	Coverage Δ
superset-frontend/src/explore/ExplorePage.tsx	0.00% <0.00%> (ø)
...t-frontend/src/explore/actions/saveModalActions.js	56.52% <0.00%> (-2.57%)	⬇️
.../explore/components/ExploreViewContainer/index.jsx	52.84% <ø> (ø)
superset-frontend/src/preamble.ts	0.00% <0.00%> (ø)
superset-frontend/src/utils/getDatasourceUid.ts	50.00% <0.00%> (-50.00%)	⬇️
superset-frontend/src/views/App.tsx	0.00% <0.00%> (ø)
...perset-frontend/src/views/CRUD/chart/ChartList.tsx	58.77% <ø> (ø)
...-frontend/src/views/CRUD/welcome/ActivityTable.tsx	42.10% <ø> (+0.72%)	⬆️
superset/views/core.py	77.38% <ø> (ø)
...rc/dashboard/components/DashboardEmbedControls.tsx	9.43% <33.33%> (ø)
... and 37 more

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[lilykuang]

LGTM