Reflect attribute/relationship capabilities in resource object schemas

[maurei]

Attribute capabilities will affect the listed properties in the attribute object of a resource object.

• The AttrCapabilities.AllowView flag must be set for a field to be present in the schema associated to a GET operation.
• The AttrCapabilities. AllowCreate flag must be set for a field to be present in the schema associated to a POST operation.
• The AttrCapabilities.AllowChange flag must be set for a field to be present in the schema associated to a PATCH operation.

These flags are currently already parsed in the custom schema generation of the current integration, but thorough testing is required.

Note that filter/sort-ability/can-include are out of scope as this requires query parameters to be implemented, see #1058.

[bkoelman]

This should include relationship capabilities as well.

[bkoelman]

This should include tests for get-only and set-only properties.

[mrnlts]

Hello, I would like to contribute to this task :)