Enhance entities:upsert to not require all mandatory fields in case of links manipulation

Who would benefit?

Everyone building integrations

What impact would it make?

Simplify preparation for using entities:upsert rest api call in case making a link between existing entities.

Why it matters?

at the moment, if

• it is links manipulation only case - which means all entities exist and the only goal of the entities:upsert call is to link the entities.
  AND

• there are more then 1 mandatory fields in the
  source entity.
  AND

• in the entities:upsert request, not all these mandatory field values are provided

the answer from the endpoint is - "Internal server error"

and to solve it - it's need to provide all the existing mandatory field values

--

yes, there is /links endpoint, but the benefit of using entities:upsert is to create all 1-* links with a single call.

How should it work?

entities:upsert endpoint is sophisticated enough to understand that the call is about links manipulation, all entities exist, so - no need to have all mandatory fieldValues.

Why is it needed?

at the moment, it's required to

1. do a request to
   model/entitytypes/{entityTypeId}/fieldtypes to understand which fieldtypes are mandatory.
   then,

2. do a request to entities:fetchdata to get the mandatory field values.
   then,

3. build entities:upsert request with the mandatory field values

all this makes an unnecessary load to the system, especially in case of massive data manipulation, like data migration or systems synchronization.