Change BasicJWTAuthenticatable usernameKey type

[Joebayld]

trafficstars

What are your thoughts on this change:

static var usernameKey: WritableKeyPath<Self, String> { get }

it is currently a KeyPath and not a WritableKeyPath.

This would make it match Vapors BasicAuthenticatable so that my model can conform to PasswordAuthenticatable and BasicJWTAuthenticatable

[proggeramlug]

I like it!

[proggeramlug]

Want to submit a PR?