Clarify requirements to meet before EntityRef is added to the Resource

[tigrannajaryan]

The following is the checklist to complete before we add the entity_refs field to the Resource.

• Functionality of id_attr_keys and descr_attr_keys fields meets our modeling needs (including confirming it can be modelled efficiently performance-wise in pdata).
• Name of the added field in the Resource is confirmed. We committed to entity_refs.
• Names of the fields in the ResourceEntityRef message are confirmed.
• The concerns with Go prototype should be resolved (@MrAlias has a concern that it is a breaking change - need to make sure it it is done in a non-breaking manner).
• Check the payload size increase.

[tigrannajaryan]

@jsuereth @dmitryax FYI

[feldentm-SAP]

I think we should also discuss

• different identification styles (pod_name vs pod_uid) as id_attr
• different identification context encoding (within cluster vs global)

Since I joined the group recently, I cannot judge the state of the two discussions. However, all options I currently see would affect how we encode entities.