BUG/MINOR: quic: fix the header protection key length dump with AWS-LC

BUG/MINOR: quic: fix the header protection key length dump with AWS-LC

Under QUIC_AEAD_API, tls_hp() returns EVP_CIPHER_CHACHA20, which is an
EVP_AEAD cast to EVP_CIPHER. Applied to it, EVP_CIPHER_key_length() reads
an unrelated field and returns 1 instead of 32, so
quic_tls_keys_hexdump() only dumps one byte of the header protection key.
quic_tls_derive_keys() has the same computation but is guarded by
 #ifdef QUIC_AEAD_API. Move that special case to a helper in quic_tls.h
and use it for both.

This only affects the trace output. Measured on AWS-LC 5.9.0: 1 instead
of 32 for ChaCha20, AES-128 and AES-256 unaffected.

Should be backported to all stable versions even if only the QUIC traces
are impacted.