Make x-hasura-allowed-roles field in JWT optional

### Is your proposal related to a problem?

Right now `x-hasura-allowed-roles` is a required field in the JWT, this makes it so you can't just choose a JWT field as a role because you need a second field with that first field in array form.

Please see [this stackoverflow post for an example](https://stackoverflow.com/questions/71864595/how-to-map-correctly-to-make-x-hasura-allowed-roles-field-an-array-using-claim).

### Describe the solution you'd like

Make `x-hasura-allowed-roles` optional in the JWT, it can default to the value of `x-hasura-default-role`

### Describe alternatives you've considered

I'm not sure of an alternative if you don't have full control over the JWT

### If the feature is approved, would you be willing to submit a PR?

I don't know Haskell well enough unfortunately

No file or test is named in the issue. Begin by tracing how JWT claims are validated and how x-hasura-default-role and x-hasura-allowed-roles are used; done means a JWT without the allowed-roles claim can use the default role without breaking existing role handling.

Written by the indexing model from the issue text.