feat: add 'dashboard.nav.right' extension to registry

[samtfm]

SUMMARY

Adds a extension point to the right side of the dashboard nav

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 #20835 (a25e0c7) into master (1101922) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #20835      +/-   ##
==========================================
+ Coverage   66.20%   66.24%   +0.03%     
==========================================
  Files        1754     1764      +10     
  Lines       66678    66970     +292     
  Branches     7049     7130      +81     
==========================================
+ Hits        44143    44363     +220     
- Misses      20738    20783      +45     
- Partials     1797     1824      +27     

Flag	Coverage Δ
javascript	52.12% <100.00%> (+0.19%)	⬆️

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

Impacted Files	Coverage Δ
...set-ui-core/src/ui-overrides/ExtensionsRegistry.ts	100.00% <ø> (ø)
...frontend/src/dashboard/components/Header/index.jsx	60.29% <100.00%> (+0.89%)	⬆️
...t-frontend/src/dashboard/containers/SliceAdder.jsx	0.00% <0.00%> (-100.00%)	⬇️
...rontend/src/dashboard/containers/DashboardGrid.jsx	0.00% <0.00%> (-100.00%)	⬇️
superset-frontend/src/featureFlags.ts	66.66% <0.00%> (-33.34%)	⬇️
...components/DashboardBuilder/DashboardContainer.tsx	40.00% <0.00%> (-32.23%)	⬇️
...nd/src/dashboard/util/activeAllDashboardFilters.ts	60.00% <0.00%> (-26.96%)	⬇️
...set-frontend/src/explore/actions/hydrateExplore.ts	42.10% <0.00%> (-19.44%)	⬇️
...lugin-chart-handlebars/src/plugin/controlPanel.tsx	50.00% <0.00%> (-16.67%)	⬇️
...c/dashboard/components/dnd/AddSliceDragPreview.jsx	50.00% <0.00%> (-10.00%)	⬇️
... and 117 more

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