Adds support for the "diff" type proxy plugin. This allows for out of process diff plugins to be used using the existing proxy plugin framework. The contrib addition makes it easier to implement such plugins, similar to the snapshotservice in contrib.

Moves the client proxy out of the client package to a package under diff, similar to our other client proxies.

A simple example is at https://gist.github.com/dmcgowan/93a19e4183493b46c5bf8cc3d5be9e59

Since we already have support for multiple differs (similar to how we have support for multiple snapshotters), configuring is easy. Just add the proxy plugin and update the diff service to use it (add it before walking to try the proxy first).

...

  [plugins."io.containerd.service.v1.diff-service"]
    default = ["proxydiff", "walking"]

...

[proxy_plugins]

  [proxy_plugins."proxydiff"]
    type = "diff"
    address = "/tmp/proxy.sock"