[Feature request] Support multiple ciphersuites in Bertie

**Is your feature request related to a problem? Please describe.**
Clients usually don't know what ciphersuites the server supports and should therefore advertise and handle multiple ciphersuites.
Servers must support multiple ciphersuites as well in order to communicate with different clients.

**Describe the solution you'd like**
Allow setting multiple ciphersuites in `client_connect` and `server_accept`.

Begin by tracing the client_connect and server_accept entry points to understand how a single ciphersuite is currently configured and negotiated. Define the expected behavior for advertising, selecting, and handling multiple ciphersuites on both sides; done means clients and servers can interoperate when their supported sets differ.

Written by the indexing model from the issue text.