Resolve CE-1774 "Modify template api again"

Whoops, types/{type_id}/preview_generation should have been a POST request since we're sending info in the body now.

Now the OpenAPI definition should be correct as well...

"/api/v1/types/{type_id}/preview_generation": {
            "post": {
                "tags": [
                    "Types"
                ],
                "summary": "Generate IOC instances preview",
                "operationId": "previewGeneration",
                "parameters": [
                    {
                        "name": "type_id",
                        "in": "path",
                        "description": "Type ID",
                        "required": true,
                        "schema": {
                            "type": "integer",
                            "format": "int64"
                        }
                    }
                ],
                "requestBody": {
                    "content": {
                        "application/json": {
                            "schema": {
                                "$ref": "#/components/schemas/PreviewGenerationRequest"
                            }
                        }
                    },
                    "required": true
                },
                "responses": {...}
            }
        },