Support foreign key relationships in GraphQL API queries

[ra0x3]

• This was originally created in an old issue Allow querying Foreign Key items and return relations as objects #136 -- but we never got too it because we had other things going on
• However, a user in Querying nested objects only provides IDs of the nested objects #452 has raised this an issue again - so it needs to be triaged
• We have support for foreign keys in our SQL backends
• However, the GraphQL API is not aware of these foreign key relationships. So when a user queries for a foreign key relationship, they just get back the reference column value as opposed to the foreign object (see Querying nested objects only provides IDs of the nested objects #452 for more info)
• We need to update our GraphQL API so that entire foreign key objects are returned

Additional context

• After taking a cursory look -- without any way to make GraphQL aware of what the foreign key relationships are, this could be a non-trivial/heavy lift
• The changes would have to be reflected in our graphql schema mod
• Should include tests