Relationship capabilities and bugfixes

[bkoelman]

Adds the equivalent of AttrCapabilities for relationships. See #1187 for design considerations.
This PR also fixes some small bugs.

Closes #1187.
Fixes #1196.
Fixes #1195.
Fixes #1198.
Fixes #1199.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated

[bkoelman]

This PR has a dependency on #1192. If that one is merged earlier, this PR needs additional changes.

[codecov]

Codecov Report

Merging #1197 (35ec18f) into master (ea3ab72) will increase coverage by 0.08%.
The diff coverage is 95.16%.

❗ Current head 35ec18f differs from pull request most recent head 4b725bd. Consider uploading reports for the commit 4b725bd to get more accurate results

@@            Coverage Diff             @@
##           master    #1197      +/-   ##
==========================================
+ Coverage   92.61%   92.69%   +0.08%     
==========================================
  Files         242      243       +1     
  Lines        7701     7803     +102     
==========================================
+ Hits         7132     7233     +101     
- Misses        569      570       +1     

Impacted Files	Coverage Δ
...Annotations/Resources/Annotations/AttrAttribute.cs	90.00% <ø> (ø)
...ns/Resources/Annotations/ResourceFieldAttribute.cs	65.00% <ø> (ø)
...ourceGenerators/TypeWithAttributeSyntaxReceiver.cs	100.00% <ø> (ø)
src/JsonApiDotNetCore/Configuration/TypeLocator.cs	89.74% <ø> (ø)
...ation/JsonConverters/WriteOnlyDocumentConverter.cs	91.66% <ø> (ø)
...Converters/WriteOnlyRelationshipObjectConverter.cs	81.25% <ø> (ø)
...piDotNetCore/Configuration/ResourceGraphBuilder.cs	97.12% <85.71%> (-1.57%)	⬇️
...re/Resources/Annotations/CapabilitiesExtensions.cs	88.00% <88.00%> (ø)
...otations/Resources/Annotations/HasManyAttribute.cs	100.00% <100.00%> (ø)
...notations/Resources/Annotations/HasOneAttribute.cs	100.00% <100.00%> (ø)
... and 16 more

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[maurei]

Reviewed extensively, work is very clean 👍