feat(auth): implement supertokens authentication abstraction

[KabinKhandThakuri]

Auth Abstraction Layer — Decouples @prefabs.tech/fastify-user from direct SuperTokens dependency by introducing a pluggable Auth Adapter interface.

What's included:

• Auth abstraction ( src/auth/ ) — AuthAdapter interface + provider registry allowing custom auth backends via registerAuthProvider()
• SuperTokens adapter — Maps SuperTokens SDK to the generic adapter; remains the default provider
• Dynamic plugin registration — Plugin now initializes via initAuth(fastify, provider) instead of hardcoded SuperTokens
• Handlers decoupled — All user handlers ( me.ts , changeEmail.ts , adminSignUp.ts , etc.) no longer import SuperTokens directly
• Cleaner types — AuthUser , User , UserCreateInput , UserUpdateInput come from the abstraction; recipe-specific types deprecated

Breaking changes:

• UserConfig.supertokens → now optional; use authProvider config to specify custom provider
• AuthUser type removed from types/user.ts — import from @prefabs.tech/fastify-user/auth
• supertokensErrorHandler , areRolesExist , createUserContext exports deprecated — use @prefabs.tech/fastify-user/supertokens sub-path
• supertokens-node moved to optional peer dependency