Support disable encryption

Support disabling encryption in trusted environments, for research, debugging, and performance profiling.

tquic and msquic support this extension: https://datatracker.ietf.org/doc/html/draft-banks-quic-disable-encryption

Start by reading the linked QUIC extension draft, then trace how quiche currently handles encryption and identify the entry points affected by allowing it to be disabled. The work is complete when trusted-environment users can disable encryption for research, debugging, and performance profiling while preserving normal encrypted behavior.

Written by the indexing model from the issue text.