[webrtc-pc] Allow adminstrative rejection of candidates (#2534)

alvestrand has just created a new issue for https://github.com/w3c/webrtc-pc:

== Allow adminstrative rejection of candidates ==
In conjunction with the portscanning issue, it was stated (https://github.com/w3c/webrtc-pc/issues/2426#issuecomment-570528505) that "I think we should state that ICE-TCP ports should not connect to ports on the Fetch "bad ports" list - https://fetch.spec.whatwg.org/#port-blocking"

There might be other reasons for administratively rejecting a candidate - for instance overuse of resources, or local policy.

This seems to deserve its own issue.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2534 using your GitHub account