UserAppMetadata is not fully typed

Current typing

```ts
export interface UserAppMetadata {
  provider?: string
  [key: string]: any
}
```


Should be (could be)
```ts
type AuthProvider = Provider & ("email" | (string & {})); // needs more work here
export interface UserAppMetadata {
  provider?: AuthProvider // assign a type
  providers?: AuthProvider[] // define a field
  [key: string]: any
}
```