Draft: private bridge pinning

as a follow up of !243 (merged) and !201 (merged): allow to pin any bridge pointing to a providers gateway. Reqiured Formatting is derived from the transports json defined in vpnweb!20

Hopping PT

         {
            "type":"obfs4-hop",
            "protocols":[ "tcp"],
            "options": {
              "endpoints": [
                {
                  "ip_address": "1.2.3.4",
                  "cert": "CERT 1"
                },
                {
                  "ip_address": "1.2.3.5",
                  "cert": "CERT 2"
                },
              ],
              "iatMode": "0",
              "port_seed": 94,
              "port_count": 100,
              "experimental": true
            }
          }

obfs4

         {
            "type":"obfs4",
            "protocols":[ "tcp"],
            "options": {
              "endpoints": [
                {
                  "ip_address": "1.2.3.4",
                  "cert": "CERT 1"
                }
              ],
              "iatMode": "0",
              "port_seed": 94,
              "port_count": 100,
              "experimental": true
            }
          }