GraphQL (part 2): Model changes and resolvers (#19486)

This PR adds the necessary resolvers to support
the graphQL front-end. Some additional methods
need to be added to the model structs to support
this. Comments have been made to clarify their purpose.

There is a slight duplication of code in the api layer.
But eventually that's going to go away when we move
away from the REST API entirely.

The schema is in api4/schema.graphqls and gets compiled
into the binary as an asset.

The GraphiQL editor url is at GET /api/v5/graphql.

Everything is behind the feature flag MM_FEATUREFLAGS_GRAPHQL.

```release-note
NONE
```

model/client4.go

@@ -46,6 +46,7 @@ const (
 
 	APIURLSuffixV1 = "/api/v1"
 	APIURLSuffixV4 = "/api/v4"
+	APIURLSuffixV5 = "/api/v5"
 	APIURLSuffix   = APIURLSuffixV4
 )