fix(ipv6): add JSON IPv6 suffix parameter (#611)

- Remove `IPV6_PREFIX` environment variable (unneeded) and remove associated code
- Update all documentation for each provider supporting IPv6
- Build IPv6 as prefix:suffix when getting it from a public IP source for each record IPv6 suffix parameter
- Automatically disable provider_ip if public ip is IPv6 and IPv6 suffix is set (they are not compatible with each other)

docs/inwx.md

@@ -13,7 +13,8 @@
       "host": "@",
       "username": "username",
       "password": "password",
-      "ip_version": "ipv4"
+      "ip_version": "ipv4",
+      "ipv6_suffix": ""
     }
   ]
 }
@@ -29,5 +30,6 @@
 ### Optional parameters
 
 - `"ip_version"` can be `ipv4` (A records), or `ipv6` (AAAA records) or `ipv4 or ipv6` (update one of the two, depending on the public ip found). It defaults to `ipv4 or ipv6`.
+- `"ipv6_suffix"` is the IPv6 interface indentifier suffix to use. It can be for example `0:0:0:0:72ad:8fbb:a54e:bedd/64`. If left empty, it defaults to no suffix and the raw public IPv6 address obtained is used in the record updating.
 
 ## Domain setup