KEM-based pre-shared-key handshakes for TLS 1.3

This document gives a construction in which (long-term) KEM public keys are used in the place of TLS PSK keys, avoiding concerns that may affect systems that use symmetric-key-based PSK, such as requiring key diversification and protection of symmetric-keys' confidentiality. This mechanism is inspired by AuthKEM (and could use AuthKEM certificate public keys for resumption), but can be independently implemented.