Support proxy on a per "replication endpoint" base in replication.

[reasonerjt]

Enterprise users usually have restrictive network policies, there can be different registries deployed in different networks that are isolated from each other.
To support replication across these registries Harbor needs to access them via different proxy. Currently, this is not possible b/c we only support configuring global proxy via environment variable of containers.

We can add more attributes to "replication endpoint" to support different proxies for different registries.