Add Context.set_sigalgs_list() and Connection.get_sigalgs()

[Jean-Daniel]

This is based on SSL_CTX_set1_sigalgs(3).

It let the client limits the set of signature algorithms that should be used by the server for certificate selection.

This extension is supported from TLS 1.2, but it is especially useful with TLS 1.3, as the new cipher list used by TLS 1.3 no longer provide this feature.