* refactor: auto get client ip from challenge response
Since `outip()` was not working properly on devices getting local IP addresses behind a router, we should refactor this.
After analyzing the auth process, it is shown that the challenge response includes key `client_ip` which is the real public IP address with key `ip` not specified in request.
- removed `outip()`
- added rsp key `ClientIP` to get client ip from challenge rsp
* style: trim code
* style: fix spelling issues
* refactor: create `portal_test.go` to handle portal tests separately
* feature: add `ResolveLocalClientIP()` and its test case
* optimize: resolve ClientIP locally when cant be get from challenge response
