Nullable reference types

[bart-degreed]

Closes #1029.

This PR annotates the codebase for use with nullable reference types (NRT).

This does not mean your API project that uses JADNC must do so too. But once you do, additional warnings will light up for possible NullReferenceExceptions and unneeded null checks in your code.

Overview of changes:

• Breaking: ResourceType (formerly ResourceContext) now provides Get methods that throw when not found and Find methods that return null when not found. For example: GetAttributeByPublicName() vs FindAttributeByPublicName(). The same applies to methods on IResourceGraph. For example: GetResourceType() vs FindResourceType().
• Breaking: An injectable dependency of type IResourceGraph was added to Controller constructors.
• Fixed: JADNC now renders the correct path in error.source.pointer for required relationships that are missing in a post/patch request body when ASP.NET ModelState validation is enabled. And it takes JsonPropertyNameAttribute from System.Text.Json into account when properties on complex attributes are named differently. For details, see ./test/JsonApiDotNetCoreTests/UnitTests/ModelStateValidation/ModelStateValidationTests.cs.
• Breaking: ASP.NET ModelState validation is now enabled by default. It can still be turned off in options. The rationale is that when using NRT, resource properties and relationships become required by default where they were optional before. When ModelState is enabled, this produces validation errors with reliable source pointers, which is more helpful to API clients than JADNC catching downstream exceptions (where the source path is lost) or even HTTP 500 errors due to SQL foreign key constraints. And even if your project is not using NRT, putting [Required] on attributes and relationships provides the same benefits.
• Fixed: JADNC now respects configured MaxModelValidationErrors (https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.mvc.mvcoptions.maxmodelvalidationerrors) in atomic:operations requests, which can be set in services.AddMvcCore() (defaults to 200).
• Documentation was added on how to deal with nullable value/reference types in resource models. Also added notes on how to map required one-to-one relationships correctly.
• Made serialization of response bodies ~10% faster by caching resource graph information that doesn't change per request.
• Fixed: Improved validation of JSON request bodies when various elements are explicitly set to null.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated
• N/A: Created issue to update Templates: {ISSUE_NUMBER}

[codecov]

Codecov Report

Merging #1095 (4f23fc2) into master (4a2abe9) will increase coverage by 0.32%.
The diff coverage is 87.47%.

❗ Current head 4f23fc2 differs from pull request most recent head bb426c6. Consider uploading reports for the commit bb426c6 to get more accurate results

@@            Coverage Diff             @@
##           master    #1095      +/-   ##
==========================================
+ Coverage   88.21%   88.54%   +0.32%     
==========================================
  Files         256      255       -1     
  Lines        6931     7086     +155     
==========================================
+ Hits         6114     6274     +160     
+ Misses        817      812       -5     

Impacted Files	Coverage Δ
...NetCoreExample/Controllers/OperationsController.cs	0.00% <0.00%> (ø)
...iDotNetCoreExample/Controllers/PeopleController.cs	0.00% <0.00%> (ø)
...ApiDotNetCoreExample/Controllers/TagsController.cs	0.00% <0.00%> (ø)
...tNetCoreExample/Controllers/TodoItemsController.cs	0.00% <0.00%> (ø)
...ples/JsonApiDotNetCoreExample/Data/AppDbContext.cs	0.00% <0.00%> (ø)
...otNetCoreExample/Definitions/TodoItemDefinition.cs	0.00% <ø> (ø)
...Examples/JsonApiDotNetCoreExample/Models/Person.cs	0.00% <0.00%> (ø)
...rc/Examples/JsonApiDotNetCoreExample/Models/Tag.cs	0.00% <0.00%> (ø)
...amples/JsonApiDotNetCoreExample/Models/TodoItem.cs	0.00% <0.00%> (ø)
src/Examples/JsonApiDotNetCoreExample/Startup.cs	0.00% <ø> (ø)
... and 202 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4a2abe9...bb426c6. Read the comment docs.