Change `signUp` signature to avoid tautology

[phoenix-ru]

          You're actually right, it's a weird implementation in https://github.com/sidebase/nuxt-auth/pull/615 which accepts two identical interfaces:

https://github.com/sidebase/nuxt-auth/blob/218a8465fd01fbf70562ab4cc1083dea3931d1a1/src/runtime/composables/local/useAuth.ts#L161

It should be just signUpOptions, will probably move this to a different breaking change

Originally posted by @phoenix-ru in https://github.com/sidebase/nuxt-auth/pull/702#discussion_r1862455689