MINOR: quic_tls: Add quic_tls_decrypt2() implementation

MINOR: quic_tls: Add quic_tls_decrypt2() implementation

This function does exactly the same thing as quic_tls_decrypt(), except that
it does reuse its input buffer as output buffer. This is needed
to decrypt the Retry token without modifying the packet buffer which
contains this token. Indeed, this would prevent us from decryption
the packet itself as the token belong to the AEAD AAD for the packet.