K8s: Edge: Resolve domain as CANDIDATE.

[winlinvip]

In English:

In CDN or Edge scenarios, the IP of the Edge needs to be returned, for example:

Browsers --WebRTC--> Edge Proxy --> K8s(Service) --> Proxy --> SRS

At this time, the address to be returned is not the K8s address, but the address closer to the user's edge.

However, this edge address is generally a domain name, and Firefox does not support domain name CANDIDATEs, so it must be resolved. The process should be like this:

1. The browser accesses the SRS API.
2. SRS resolves the Edge domain name, and according to the client IP, resolves the nearest IP access point address.
3. The resolved IP is used as a CANDIDATE and returned to the browser via SDP.
4. The browser implements media access as shown in the diagram above.

Note: The Proxy in front of SRS is optional. After supporting the cluster, Proxy is not needed. In simple scenarios, Proxy is more suitable. See #3138

[winlinvip]

Support for retaining domain names has been added, so non-Firefox scenarios can be resolved by the client.