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/utils.go

@@ -132,6 +132,24 @@ func (m StringMap) Value() (driver.Value, error) {
 	return string(j), err
 }
 
+func (StringMap) ImplementsGraphQLType(name string) bool {
+	return name == "StringMap"
+}
+
+func (m StringMap) MarshalJSON() ([]byte, error) {
+	return json.Marshal((map[string]string)(m))
+}
+
+func (m *StringMap) UnmarshalGraphQL(input interface{}) error {
+	json, ok := input.(map[string]string)
+	if !ok {
+		return errors.New("wrong type")
+	}
+
+	*m = json
+	return nil
+}
+
 func (si *StringInterface) Scan(value interface{}) error {
 	if value == nil {
 		return nil