Revive PR: Add Application Insights HealthCheck

[cieciurm]

What this PR does / why we need it:
This PR revives #740

Which issue(s) this PR fixes:

Special notes for your reviewer:

• Merged from master
• Code updated
• Tests updated
• GitHub actions updated

Does this PR introduce a user-facing change?:

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]

Hi @sungam3r,
It's ready for review.

[sungam3r]

Please update root readme with new package.

[cieciurm]

Oops, my bad. It's been updated now - cc @sungam3r

[codecov-commenter]

Codecov Report

Merging #1891 (9a8f4ed) into master (7dc61b4) will decrease coverage by 0.43%.
The diff coverage is 36.84%.

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@@            Coverage Diff             @@
##           master    #1891      +/-   ##
==========================================
- Coverage   66.93%   66.50%   -0.43%     
==========================================
  Files         228      229       +1     
  Lines        8135     8097      -38     
  Branches      573      577       +4     
==========================================
- Hits         5445     5385      -60     
- Misses       2545     2568      +23     
+ Partials      145      144       -1     

Flag	Coverage Δ
AzureApplicationInsights	14.79% <36.84%> (?)

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

Impacted Files	Coverage Δ
...I/Core/HostedService/HealthCheckReportCollector.cs	65.60% <ø> (-0.19%)	⬇️
...ionInsights/AzureApplicationInsightsHealthCheck.cs	23.40% <23.40%> (ø)
...ApplicationInsightsHealthCheckBuilderExtensions.cs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[sungam3r]

I published preview version https://www.nuget.org/packages/AspNetCore.HealthChecks.AzureApplicationInsights/7.0.0-rc2.2.

Bad news - I accidentally published release version because of wrong tag in csproj, see 60cedfb.