Minor serialization tweaks

[bkoelman]

The initial goal of this PR was to activate the System.Text.Json source generator. It is documented to improve performance and use less memory.

After making the change, I wasn't able to see improvements in benchmarks. Narrowing down the benchmark resulted in a performance gain of 2 microseconds and 838 bytes per request. Given all the trouble we've had in the past with source generators (bugs in the .NET SDK, increased build times, Resharper poorly supporting their use), I don't think it's worth making the change, so I reverted that commit. It's still in the PR commit history, for future reference.

This PR is still useful to merge because it also contains:

• Remove unused parameters in a private method
• Rename private Attribute class to match with .NET naming conventions
• Make a private lazy member read-only and avoid potentially multiple initializations at startup
• Use [JsonPropertyOrder], which enabled to simplify the JSON:API object model a bit
• Tweaks in benchmarks project

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

Merging #1170 (f760e8f) into master (0185311) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1170      +/-   ##
==========================================
- Coverage   92.58%   92.57%   -0.01%     
==========================================
  Files         241      242       +1     
  Lines        7700     7694       -6     
==========================================
- Hits         7129     7123       -6     
  Misses        571      571              

Impacted Files	Coverage Δ
...otNetCore/Serialization/Objects/AtomicReference.cs	100.00% <ø> (ø)
.../Serialization/Objects/ResourceIdentifierObject.cs	100.00% <ø> (ø)
...DotNetCore/Serialization/Objects/ResourceObject.cs	100.00% <ø> (ø)
...tNetCore/Serialization/Objects/SingleOrManyData.cs	100.00% <ø> (ø)
...Core/Serialization/Request/Adapters/BaseAdapter.cs	100.00% <ø> (ø)
...zation/Request/Adapters/ResourceIdentityAdapter.cs	100.00% <ø> (ø)
...n/Request/Adapters/ResourceIdentityRequirements.cs	100.00% <ø> (ø)
.../JsonApiDotNetCore/Configuration/JsonApiOptions.cs	100.00% <100.00%> (ø)
.../JsonApiDotNetCore/Middleware/JsonApiMiddleware.cs	97.77% <100.00%> (ø)
...tNetCore/Queries/Internal/Parsing/IncludeParser.cs	95.83% <100.00%> (ø)
... and 2 more

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