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

[egregors]

refs:

• [BUG] Username must currently match WebAuthnId of user #27 (comment)
• 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