We create the internal/ssl.h header file and move the typedef for
ossl_msg_cb. This is needed by both the QUIC code (which generally doesn't
include ssl_local.h) and the rest of libssl.
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20914)
