[NEW] Support dual certificates

[pkhartsk]

The problem/use-case that the feature addresses

Right now, it's impossible to start a valkey server that prefers Post-quantum cryptography (PQC) ML-DSA certificates but falls back to RSA for clients that don't support the former.

An example thereof is: using redis-cli major version 7 (does not support PQC algorithms) with valkey-server major version 8 does work with RSA certificates, but not with ML-DSA, which results in

Could not connect to Redis at <ip>:6379: SSL_connect failed: ssl/tls alert handshake failure

and

Error accepting a client connection: error:0A000076:SSL routines::no suitable signature algorithm (addr=<ip>:38508 laddr=<ip>:6379)

Description of the feature

An ability to specify additional certificate/key files in the config, similar as in httpd, which also uses OpenSSL to handle TLS connections. openssl s_server itself has -dcert and -dkey flags in addition to -cert and -key for this use case.

Alternatives you've considered

I'm not sure there is an alternative.

Additional information

Hopefully this shouldn't be too hard to implement, just changing how the program interacts with OpenSSL.

[pkhartsk]

I looked into it and it should just be adding an extra SSL_CTX_use_certificate_chain_file() and SSL_CTX_use_PrivateKey_file() calls, along with error handling. The only thing is that it seems impossible are keys with different passphrases (having the same one is fine), but from what I can see other servers also don't support multiple passphrases (or they don't support them at all).
Willing to implement this as described above, an alternative being having a tls-cert-files config option with a colon-separated list of certificate files, which should be fine as long as each one has a different type. Any maintainer that can look into this?