Use OpenSSL with -sigalgs option to obtain server's certificate

[dcooper16]

Describe your changes

When $OPENSSL s_client supports the "-sigalgs" option, get_server_certificate() uses $OPENSSL rather than tls_sockets() to obtain the server's certificate, but only for certificates with RSA and ECDSA public keys.

With OpenSSL 3.5 and newer the list command can be used to get a list of supported TLS signature algorithms.

With this PR, if OpenSSL 3.5 or newer is being used, the list of supported TLS signature algorithms is obtained and get_server_certificate() uses $OPENSSL s_client rather than tls_sockets() whenever $OPENSSL supports the relevant signature scheme.

In addition to making the code a bit faster, this may be helpful if a server has a certificate with an SM2 public key and it only supports curveSM2 for key exchange, since tls_sockets() can not decrypt server responses if curveSM2 is used.

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md

[drwetter]

Thanks!