[FormState] Extend the return type of the submt handler.

[mehdi-loup]

The problem

In order to be able to return both void and RemoteError[] in an
async method, the 2 types needs to be lifted within a single Promise.
This PR extends the union of the SubmitHandler return type.

Example of use

In the render:

  <FormState onSubmit={this.handleSubmit} />

In the class:

private async handleSubmit() {
  const {data} = await mutation();
   if (error) { 
    return [{message: error.message}]
  }
  return;

Because handleSubmit being async, it would wrap all possible return type within one single promise, we need to the onSubmit prop of FormState to account for that too.