custom form definition

```yaml
template:
  inputs:
    username:
      type: string
    password:
      type: string
    title:
      type: string
    body:
      type: string
  
  form:
    items:
    - input:
        ref: username
    - input:
        ref: password
    - group:
        title: Advanced
        items:
        - input:
            ref: title
        - input:
            ref: body
```