feat: add Session Transfer Token support for CTE impersonation via session transfer

[kishore7snehil]

📋 Changes

This PR surfaces Custom Token Exchange (CTE) Impersonation via Session Transfer (RFC 8693) through the auth0-fastapi wrapper SDK. The heavy lifting lives in auth0-server-python; this SDK adds thin AuthClient wrappers so a FastAPI app can exchange a subject token for a short-lived, single-use Session Transfer Token (STT) that redeems into an impersonated web session on a target app - logging an agent in as a customer with the agent recorded in the act claim. The initiator mints the STT and builds the redirect; the target needs no new SDK code, since the mounted /auth/login route already forwards session_transfer_token to /authorize.

✨ Features

• AuthClient.request_session_transfer_token delegates to the underlying client to perform the CTE exchange against the urn:{domain}:session_transfer audience and returns a SessionTransferTokenResult
• AuthClient.build_session_transfer_redirect returns the redirect URL that hands the STT to the target app's login URL, forwarding organization when present
• The target (redeeming) side needs no new code: the mounted /auth/login route already forwards session_transfer_token (and organization) through to /authorize

🔧 API Changes

• New method request_session_transfer_token(subject_token, subject_token_type, actor_token=None, actor_token_type=None, scope=None, organization=None, store_options=None) -> SessionTransferTokenResult
• New method build_session_transfer_redirect(target_login_url, result, organization=None) -> str
• Bumped the auth0-server-python dependency to >=1.0.0b14, which provides the STT surface, the SessionTransferTokenResult model, and the ACTOR_UNAVAILABLE / SETACTOR_REQUIRED / SESSION_TRANSFER_DISABLED error codes

📖 Documentation

• Added an "Impersonation via Session Transfer (STT)" section to examples/CustomTokenExchange.md covering the FastAPI wrappers, how the flow maps onto the mounted routes, the redirect-URL security note, and the STT error codes, pointing to the auth0-server-python guide for the underlying protocol detail
• Linked the new section from the Custom Token Exchange feature list in README.md

🧪 Testing

• This change adds test coverage
• This change has been tested on the latest version of the platform/language

Contributor Checklist

• I agree to adhere to the Auth0 General Contribution Guidelines.
• I agree to uphold the Auth0 Code of Conduct.