fix(dealers): exclude unwanted fields from API

[Fenrikur]

• AttendeeNickname should only be exposed via DisplayNameOrAttendeeNickname if DisplayName is not set.
• RegistrationNumber should never be exposed.

[maakinoh]

JsonIgnore will result in an database when creating new dealer entries error since both fields are still set as Required.

Therefore, the POST API endpoint /Api/Dealers cant be used anymore for creating new dealer entries.

Since dealers are created in a separate job this should still be fine to merge but the POST endpoint /Api/Dealers could be documented accordingly

[Fenrikur]

JsonIgnore will result in an database when creating new dealer entries error since both fields are still set as Required.

Therefore, the POST API endpoint /Api/Dealers cant be used anymore for creating new dealer entries.

Since dealers are created in a separate job this should still be fine to merge but the POST endpoint /Api/Dealers could be documented accordingly

Thank you for noticing! Didn't think about that.
The API in question is currently not being used and will have to undergo some changes for its intended use in EF29 anyways, so this should not be much of a problem. Dealers are currently created by an import job directly accessing the services internally.
I've added TODO comments and temporarily set default values for the two required fields that are no longer exposed on the API just to not break things though. (=