[MD] Add dropdown header to data source single selector

[zhongnansu]

Description

• Add dropdown header to data source single selector. This header includes
  • a counter showing the count of datasource options avaliable
  • a link with text "Manage", that honors SPA(single page application) to redirect to data source management page
• Fix bug of opensearch cluster group label being rendered multiple times when toggling between options
• Add and update related unit tests

Issues Resolved

#6429

Screenshot

iShot_2024-04-12_11.18.17.mp4

Testing the changes

Check List

• All tests pass
  • yarn test:jest
  • yarn test:jest_integration
• New functionality includes testing.
• New functionality has been documented.
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

[zhongnansu]

@xeniatup Hi Xenia, could you also help review the video and screenshot I attached, to make sure this is the correct UI/UX? Thanks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.62%. Comparing base (e785fd3) to head (59d7fe8).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6431      +/-   ##
==========================================
+ Coverage   49.55%   54.62%   +5.06%     
==========================================
  Files        2670     2276     -394     
  Lines       54290    44615    -9675     
  Branches     8878     8294     -584     
==========================================
- Hits        26906    24372    -2534     
+ Misses      25720    18480    -7240     
- Partials     1664     1763      +99     

Flag	Coverage Δ
Linux_3	45.16% <100.00%> (?)
Linux_4	34.93% <26.66%> (?)
Windows_1	?
Windows_3	?

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[Flyingliuhub]

Should we use activeDataSourceCount != null to check undefined and null both?

[zhongnansu]

since bother activeCount and totalCount should be coming from somOptionsArray.length(), Since .length on an array in JavaScript always returns a non-negative integer, we should be good to not do null check here

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch-Dashboards/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch-Dashboards/backport-2.x
# Create a new branch
git switch --create backport/backport-6431-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 a1c8b8c437e34d186f3bc44436f71b62371432f1
# Push it to GitHub
git push --set-upstream origin backport/backport-6431-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch-Dashboards/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-6431-to-2.x.