feat: Add MFA (Multi-Factor Authentication) API support

[subhankarmaiti]

Adds full MFA API support to auth0-server-python, enabling developers to integrate multi-factor authentication flows into their applications.

New Features

MfaClient — accessible via server_client.mfa

• list_authenticators(options) — List enrolled authenticators for a user (OTP, OOB/SMS, OOB/Email).
• enroll_authenticator(options) — Enroll a new authenticator (OTP, SMS, or Email).
• delete_authenticator(options) — Remove an enrolled authenticator.
• challenge_authenticator(options) — Trigger an MFA challenge (e.g., send an SMS code).
• verify(options) — Verify an MFA challenge using OTP, OOB, or a recovery code.
• encrypt_mfa_token(...) / decrypt_mfa_token(...) — Securely encrypt/decrypt the mfa_token for safe round-tripping through the client (JWE with TTL expiry).

Automatic mfa_required Detection

• get_access_token() and get_token_by_refresh_token() now detect mfa_required responses from Auth0 and raise MfaRequiredError with an encrypted mfa_token, enabling seamless step-up authentication flows.

Usage Example

from auth0_server_python import ServerClient

client = ServerClient(domain="...", client_id="...", client_secret="...", secret="...")

# MFA is triggered automatically during token retrieval
try:
    token = await client.get_access_token()
except MfaRequiredError as e:
    # e.mfa_token is encrypted and safe to pass to the frontend
    mfa_token = e.mfa_token

# Challenge + verify flow
challenge = await client.mfa.challenge_authenticator({
    "mfa_token": raw_token,
    "challenge_type": "oob",
    "authenticator_id": "auth|123"
})

result = await client.mfa.verify({
    "mfa_token": raw_token,
    "oob_code": challenge.oob_code,
    "binding_code": user_input
})
# result.access_token is now available