[BUG] Respect user.id | name | displayName in traditional auth flow

[egregors]

refs:

• https://github.com/egregors/passkey/issues/27#issuecomment-2295296554
• https://www.corbado.com/blog/webauthn-user-id-userhandle

Goal flow should be look like:

• BeginRegistration
  • client:
  • user provides user.name
  • back:
  • get user by username (to prevent duplicates)
  • create new user
    • generate user.id
    • put user.name
    • get options and session from webauthn
  • gen session key

Than means in the next version I'll drop GetOrCreateUser(userID string) User from the UserStore interface and replace it more usual GetUser(userName string) *User

We can also add some UI to define displayName. But idk do we really need it in _example. Will see.

[egregors]

rel: #36