Adds support for ETag header in GET and HEAD requests

[bart-degreed]

See documentation updates for details.

Closes #933.

[codecov]

Codecov Report

Merging #998 (b0ed62c) into master (d033166) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #998      +/-   ##
==========================================
+ Coverage   91.26%   91.32%   +0.06%     
==========================================
  Files         281      282       +1     
  Lines        7429     7484      +55     
==========================================
+ Hits         6780     6835      +55     
  Misses        649      649              

Impacted Files	Coverage Δ
...JsonApiDotNetCore/Controllers/JsonApiController.cs	80.00% <ø> (ø)
...NetCore/Configuration/JsonApiApplicationBuilder.cs	100.00% <100.00%> (ø)
.../JsonApiDotNetCore/Middleware/JsonApiMiddleware.cs	100.00% <100.00%> (ø)
...c/JsonApiDotNetCore/Serialization/ETagGenerator.cs	100.00% <100.00%> (ø)
...c/JsonApiDotNetCore/Serialization/JsonApiWriter.cs	98.43% <100.00%> (+1.00%)	⬆️
...e/Configuration/ResourceDescriptorAssemblyCache.cs	100.00% <0.00%> (ø)
...DotNetCore/Configuration/ServiceDiscoveryFacade.cs	97.29% <0.00%> (+0.04%)	⬆️

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