Improved ModelState validation

[bart-degreed]

We currently have a solution in place to enable partial patch, combined with required fields. Users need to add [IsRequired] instead of the built-in [Required] attribute on their resource properties.

Although it works, downsides of this approach are:

• Because IsRequired derives from Required, it is picked up by EF Core to generate non-null database columns. But other tools may not recognize it.
• At the time that validation logic inside [IsRequired] runs, we have no way to know where in the object graph we are. This is important because on relationship updates, only their IDs are sent, so we must turn off some checks. The way this is implemented now is kinda hacky, but seems to work (even with self-referencing entities in basic scenarios).
• The current solution cannot work when we need to validate multiple resources per request (atomic operations).

A quick search made us suspect there's a better way, by interacting with the validation process instead of inventing our own attribute. See https://stackoverflow.com/questions/36982370/is-it-possible-to-use-a-custom-modelstate-validation-provider-in-asp-net-core for details.

This issue tracks the work to deprecate [IsRequired] and plug into the ModelState validation process instead.