SSL: add $ssl_sigalgs variable

SSL: add $ssl_sigalgs variable

The new $ssl_sigalgs variable lists all signature algorithms
advertised by the client in its ClientHello message,
colon-separated, in analogy to $ssl_ciphers and $ssl_curves.

On OpenSSL 4.0+, SSL_get0_sigalg() is used, which returns proper
TLS scheme names (e.g. "rsa_pkcs1_sha256", "ecdsa_secp256r1_sha256")
and falls back to "0xHHHH" hex for unknown schemes.

On OpenSSL 1.0.2+, SSL_get_sigalgs() is used.  Since OBJ_nid2sn()
on the combined psignhash NID is not injective across TLS
SignatureScheme codes (e.g. 0x0403 and 0x081a both yield
"ecdsa-with-SHA256"), raw SignatureScheme codes are reported to
avoid ambiguity and to match the hex fallback format of
SSL_get0_sigalg().

With older versions, as well as BoringSSL, LibreSSL, and AWS-LC,
the variable is empty.