Add TLS encryption

[mahdi-orbitalize]

What I did:

• consensus.go was updated to enable client-side (handled by rafthttp if we provide a transport.TLSInfo when constructing a rafthttp.transport) and server-side (handled by extracting a tls.Config from the transport.TLSInfo and passing it to the server at construction plus using ListenAndServeTLS instead of ListenAndServe)
• gen-cert.sh was introduced to make certificates (with the number of nodes as a parameter). Note that it's made to work for local setups (i.e. address is localhost or 127.0.0.1). I also provided example certificates to save the trouble of generating them.
• params.go was updated to support passing the paths to the TLS certificates as a parameter. It also enforces the use of HTTPS instead of HTTP for the peers' URLs. Also, more tests were added to params_test.go to test the new features.
• start_cluster.sh was created to quickly test a 3-node setup with TLS and easily inspect logs.

How I tested it:

• Verified that logs where similar when trying with/without encryption (and that it fails when it's supposed to fail)