DocumentDb - Add option to check database and collection

[cieciurm]

What this PR does / why we need it:

• Extends DocumentDb Health Check with option to check against provided database and collection
• When both provided then Health Check calls ReadDocumentCollectionAsync(...)

Which issue(s) this PR fixes:
closes #333

Special notes for your reviewer:

• Registration test added

Does this PR introduce a user-facing change?: No.

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• Code compiles correctly
• Created/updated tests
• Unit tests passing
• End-to-end tests passing
• Extended the documentation
• Provided sample for the feature

[cieciurm]

Changes applied, thanks for the review

cc: @sungam3r

[cieciurm]

Updated, ready now.

cc: @sungam3r

[codecov-commenter]

Codecov Report

Merging #1985 (8ee009a) into master (1a9df92) will decrease coverage by 0.22%.
Report is 2 commits behind head on master.
The diff coverage is 31.25%.

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@@            Coverage Diff             @@
##           master    #1985      +/-   ##
==========================================
- Coverage   66.33%   66.11%   -0.22%     
==========================================
  Files         253      251       -2     
  Lines        8717     8720       +3     
  Branches      626      625       -1     
==========================================
- Hits         5782     5765      -17     
- Misses       2772     2793      +21     
+ Partials      163      162       -1     

Flag	Coverage Δ
Dapr	?
DocumentDb	14.64% <31.25%> (+0.46%)	⬆️

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

Files Changed	Coverage Δ
...c/HealthChecks.DocumentDb/DocumentDbHealthCheck.cs	20.00% <21.42%> (-5.00%)	⬇️
src/HealthChecks.DocumentDb/DocumentDbOptions.cs	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

[sungam3r]

Thanks.

[cieciurm]

Thanks @sungam3r for review and explanations!