Forms REST API missing Form Instance Update

**Describe the bug**
The POST body to `/form-api/form/form-instances` accepts a formInstanceId, but it is not used. The Forms Java API provides provides both `createFormInstance` and `saveFormInstance` methods, but the REST API only uses the `createFormInstance` methods.

**Expected behavior**
Standard REST practice would be to have PUT endpoint to update an existing FormInstance, but overloading the POST to save instead of creating when a formInstanceId is possible.

**Additional context**
The Task app uses the `saveFormInstance` when updating a task with a form, implying that updating existing is the expected behavior.