TLS module refactoring

[BiagioFesta]

--- This breaks API compatibility ---

Before

• Certificate was the union of a certificate chain and a private key.

use wtransport::Certificate;

 let config = ServerConfig::builder()
        .with_bind_default(4433)
        .with_certificate(certificate)
        .build();

After

Now stuff should be clearer.

• Certificate is just a single x509 certificate (which implements Debug)
• Private Key is a single private key.
• Identity is aggregation of multiple certificates (certificate chain) and a private key
  • Identity implements Debug without leaking the private key.

use wtransport::Identity;

 let config = ServerConfig::builder()
        .with_bind_default(4433)
        .with_identity(&identity)
        .build();