Generated PartialUpdate and Response protos should not have pk as optional

[legau]

When generating protos with a PartialUpdate we get :

message SomethingPartialUpdateRequest {
    optional string uuid = 1;
    ...
}
message SomethingResponse {
    optional string uuid = 1;
    ...
}

These pk fields should be required

[AMontagu]

As discussed we keep otional for PartialUpdate to keep same logic used for the create/update message.
If someone reaaly want to have the lookup field of the partial update not optional we recommend to manually do this logic into a GenerationPlugin