Add file health check similar to folder health check

[michaelmairegger]

This health check allows to check fo existing files similar to FolderHealthCheck

services.AddHealthChecks()
    .AddFile(setup => setup.AddFile("FileUnderTest")), tags: new string[] { "file" });

[sungam3r]

@michaelmairegger Thanks. I'm fine to merge after you add xml comments for new public APIs and some notes in README.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Comparison is base (c000704) 69.24% compared to head (55eb377) 69.35%.
Report is 19 commits behind head on master.

Files	Patch %	Lines
src/HealthChecks.System/FileHealthCheck.cs	80.00%	4 Missing and 1 partial ⚠️
src/HealthChecks.System/FileHealthCheckOptions.cs	60.00%	4 Missing ⚠️
...ncyInjection/SystemHealthCheckBuilderExtensions.cs	87.50%	0 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2027      +/-   ##
==========================================
+ Coverage   69.24%   69.35%   +0.10%     
==========================================
  Files         176      178       +2     
  Lines        4582     4633      +51     
  Branches      445      452       +7     
==========================================
+ Hits         3173     3213      +40     
- Misses       1305     1313       +8     
- Partials      104      107       +3     

Flag	Coverage Δ
System	42.56% <78.43%> (+5.28%)	⬆️

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[michaelmairegger]

@sungam3r The xml comment is already existing. The new api for files is commented the same as the existing api for folders.