[Backport 2.x] Add default icon for selectable component and make sure the default datasource shows automatically

[opensearch-trigger-bot]

Backport 726fb0e from #6327.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 67.34%. Comparing base (f838b25) to head (cd93ed2).

Files	Patch %	Lines
.../data_source_selectable/data_source_selectable.tsx	80.76%	3 Missing and 2 partials ⚠️
...rc/plugins/data_source_management/public/plugin.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #6346      +/-   ##
==========================================
- Coverage   67.34%   67.34%   -0.01%     
==========================================
  Files        3344     3344              
  Lines       64867    64874       +7     
  Branches    10395    10396       +1     
==========================================
+ Hits        43684    43687       +3     
- Misses      18654    18658       +4     
  Partials     2529     2529              

Flag	Coverage Δ
Linux_1	31.30% <ø> (ø)
Linux_2	55.39% <ø> (ø)
Linux_3	45.10% <78.57%> (+0.01%)	⬆️
Linux_4	?
Windows_1	31.33% <ø> (ø)
Windows_2	55.35% <ø> (ø)
Windows_3	45.10% <78.57%> (+<0.01%)	⬆️
Windows_4	35.20% <0.00%> (-0.01%)	⬇️

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