Client-generated IDs per resource type

[bkoelman]

Obsoletes the boolean IJsonApiOptions.AllowClientGeneratedIds in favor of the new IJsonApiOptions.ClientIdGeneration, which is an enumeration of { Forbidden, Allowed, Required }. An error is now produced when a create-resource request omits the ID while configured to be required.

Client-generated IDs can be overruled per resource type, for example:

[Resource(ClientIdGeneration = ClientIdGenerationMode.Required)] public class Article : Identifiable<Guid> { // ... }

Closes #1299

QUALITY CHECKLIST

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

[codecov]

Codecov Report

Merging #1305 (abf8ad3) into master (0ce680c) will decrease coverage by 0.03%.
The diff coverage is 92.50%.

@@ Coverage Diff @@ ## master #1305 +/- ## ========================================== - Coverage 93.02% 93.00% -0.03%  ========================================== Files 268 268 Lines 8777 8793 +16 ========================================== + Hits 8165 8178 +13  - Misses 612 615 +3 

Files Changed	Coverage Δ
...DotNetCore/Resources/ResourceDefinitionAccessor.cs	100.00% <ø> (ø)
.../JsonApiDotNetCore/Configuration/JsonApiOptions.cs	96.42% <33.33%> (-3.58%)	⬇️
.../Resources/Annotations/ResourceAttribute.shared.cs	83.33% <66.66%> (-16.67%)	⬇️
...tNetCore.Annotations/Configuration/ResourceType.cs	96.82% <100.00%> (+0.05%)	⬆️
...piDotNetCore/Configuration/ResourceGraphBuilder.cs	97.17% <100.00%> (+0.04%)	⬆️
...n/Request/Adapters/AtomicOperationObjectAdapter.cs	98.57% <100.00%> (-0.04%)	⬇️
.../DocumentInResourceOrRelationshipRequestAdapter.cs	96.42% <100.00%> (-0.24%)	⬇️
...zation/Request/Adapters/RelationshipDataAdapter.cs	100.00% <100.00%> (ø)
...zation/Request/Adapters/ResourceIdentityAdapter.cs	100.00% <100.00%> (ø)
...n/Request/Adapters/ResourceIdentityRequirements.cs	100.00% <100.00%> (ø)
... and 1 more