[Multi Datasource] Unify getDefaultDataSourceId and export

[raintygao]

Description

1. Add getDefaultDataSourceId and getDefaultDataSourceId$ function, unify in DSM and export in setup for other plugins to use
2. Unify default data source ui settings id

Context

opensearch-project/dashboards-assistant#191 (comment)

Testing the changes

Changelog

• refactor: Unify getDefaultDataSourceId and export

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

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 67.41%. Comparing base (e2c0df2) to head (61dfa8c).

Files	Patch %	Lines
...c/components/edit_data_source/edit_data_source.tsx	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6843      +/-   ##
==========================================
- Coverage   67.43%   67.41%   -0.02%     
==========================================
  Files        3444     3444              
  Lines       67834    67839       +5     
  Branches    11031    11033       +2     
==========================================
- Hits        45741    45736       -5     
- Misses      19427    19438      +11     
+ Partials     2666     2665       -1     

Flag	Coverage Δ
Linux_1	33.07% <ø> (ø)
Linux_2	55.12% <ø> (ø)
Linux_3	45.25% <94.44%> (+0.01%)	⬆️
Linux_4	34.82% <7.69%> (-0.01%)	⬇️
Windows_1	33.09% <ø> (-0.03%)	⬇️
Windows_2	55.09% <ø> (ø)
Windows_3	45.26% <94.44%> (+<0.01%)	⬆️
Windows_4	34.82% <7.69%> (-0.01%)	⬇️

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[BionIT]

Could we add any tests that have been done for this change?

[raintygao]

Could we add any tests that have been done for this change?

Sure, updated.

[SuZhou-Joe]

How can other plugins use this method? I thought it will be export through data source service.

[raintygao]

How can other plugins use this method? I thought it will be export through data source service.

Just discussed with @SuZhou-Joe @wanglam offline, we could return these util functions in setup. So that other plugins can use these functions through optionalPlugin instead of required.