Introduce the PACKET_msg_start() function

Introduce the PACKET_msg_start() function

This gives us the start of the buffer in use for the PACKET.

We then use this information when calculating the TLS PSK binder.
Previously we were assuming knowledge about where the buffer starts.
However, with ECH, we may be using a different buffer to normal so it is
better to ask the PACKET where the start of the buffer is.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27776)