[Backport 2.x] Add saved objects service status api

[opensearch-trigger-bot]

Backport d8c7f48 from #4696.

[codecov]

Codecov Report

Merging #4708 (a374ae8) into 2.x (5a58b24) will increase coverage by 0.00%.
The diff coverage is 87.50%.

@@           Coverage Diff           @@
##              2.x    #4708   +/-   ##
=======================================
  Coverage   66.11%   66.11%           
=======================================
  Files        3315     3315           
  Lines       63892    63907   +15     
  Branches    10123    10126    +3     
=======================================
+ Hits        42239    42254   +15     
  Misses      19168    19168           
  Partials     2485     2485           

Flag	Coverage Δ
Linux_1	34.77% <ø> (ø)
Linux_2	55.10% <87.50%> (+0.02%)	⬆️
Linux_3	43.06% <0.00%> (-0.03%)	⬇️
Linux_4	35.08% <0.00%> (-0.02%)	⬇️
Windows_1	34.78% <ø> (ø)
Windows_2	55.06% <87.50%> (+0.02%)	⬆️
Windows_3	43.07% <0.00%> (-0.03%)	⬇️
Windows_4	35.08% <0.00%> (-0.02%)	⬇️

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Files Changed	Coverage Δ
src/core/server/legacy/legacy_service.ts	74.24% <0.00%> (-1.15%)	⬇️
src/core/server/plugins/plugin_context.ts	45.45% <ø> (ø)
...server/saved_objects/saved_objects_service.mock.ts	100.00% <ø> (ø)
...core/server/saved_objects/saved_objects_service.ts	88.09% <93.33%> (+2.38%)	⬆️

... and 1 file with indirect coverage changes