fix: create hierarchical input messages for write functions

[RutZap]

When creating write messages with inline fields; e.g.:

// keel schema
write testAction(id, name?, relation.id?) returns (SomeResponse)

we should create hierarchical input messages for any models field (relation.id) e.g.:

// json schema
{
 "type": "object",
 "properties": {
   "id": { "type": "string" },
   "name": { "type": "string" },
   "relation": { "$ref": "#/components/schemas/TestActionRelationInput" }
 },
"components": {
   "schemas": {
     "TestActionRelationInput": {
       "type": ["object", "null"],
       "properties": { "id": { "type": "string" } },
       "additionalProperties": false
     }
   }
 }