Support extra domain names for proxy.

Make it possible to specify directory where proxy will look up for
extra certificates. Proxy will iterate through subdirs of that directory
and load `key.pem` and `cert.pem` files from each subdir. Certs directory
structure may look like that:

  certs
  |--example.com
  |  |--key.pem
  |  |--cert.pem
  |--foo.bar
     |--key.pem
     |--cert.pem

Actual domain names are taken from certs and key, subdir names are
ignored.

proxy/src/proxy/tests.rs

@@ -54,9 +54,11 @@ fn generate_tls_config<'a>(
             .with_single_cert(vec![cert], key)?
             .into();
 
+        let common_names = Some([common_name.to_owned()].iter().cloned().collect());
+
         TlsConfig {
             config,
-            common_name: Some(common_name.to_string()),
+            common_names,
         }
     };