Support SLH-DSA server keys

[dcooper16]

This PR adds support for server certificates that have SLH-DSA public keys. The code points come from https://datatracker.ietf.org/doc/draft-reddy-tls-slhdsa/. These code points were added to OpenSSL 4.1.0-dev by openssl/openssl#31248.

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[drwetter]

Thanks a lot, @dcooper16 !!

The elif statements around L8570 and L9445 became a little ugly, if you want and have the opportunity a case statement would be more clean.