Provide QUIC encryption levels.

Provide QUIC encryption levels.

QUIC wants to know what "encryption level" handshake messages should be
sent at. Provide an ssl_encryption_level_t enum (via BoringSSL) that
defines these (of course quictls decided to make this an
OSSL_ENCRYPTION_LEVEL typedef, so provide that as well).

Wire these through to tls13_record_layer_set_{read,write}_traffic_key() so
that they can be used in upcoming commits.

ok tb@