Validation function not called for "relation" custom fields

[michaelbromley]

Describe the bug
If a validate function is defined for a custom field of type 'relation' it does not get called when creating or updating an entity with that custom field.

Expected behavior
The validate function should be invoked.

Environment (please complete the following information):

• @vendure/core version: 1.1.2
• Nodejs version: any
• Database (mysql/postgres etc): any

Additional context
The root cause is that relation custom fields are named <name>Id rather than just <name> as is the case with all other types. This difference is overlooked in the validation logic here so those fields do not get validated.