PATCHing entities validates the entire entity, also unmodified fields, so unmodified fields can throw validation errors

Problem/Motivation

Discovered while working on #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method. See #2737719-91: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method.

Updating an entity via REST fails when the entity has a certain field value that the current user may not set, even if the PATCH request does NOT modify that field.

For example: node body uses a certain format that user A is not allowed to use, but when user A PATCHes only the node title, they still get a validation error.

Proposed resolution

If a REST request sends a field that the user has access to but is unchanged from the current field value the validation should not be run on this field.

1. Add \Drupal\rest\Plugin\rest\resource\EntityResourceValidationTrait::validateWithFilteredFields().
2. Let EntityResource::patch() call EntityResourceValidationTrait::validateWithFilteredFields() rather than EntityResourceValidationTrait::validate().
3. (We still need EntityResourceValidationTrait::validate() for POSTing.)

Note that EntityResourceValidationTrait is @internal, so this is definitely not a BC break.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.