liaison, resources: provide the skeleton of Resources API (istio#365)

This provides the skeleton for `ResourcesService`. This supersedes tetrateio/tetrate#326.

An example of calling `SetResource` showing the involved components is as follows:

![image](https://user-images.githubusercontent.com/73152/51816698-66994580-22fa-11e9-8247-5a92395ee236.png)

Mirrored from https://github.com/tetrateio/tetrate @ 1c1e5710727edb22a1a853b6b06691b974a760c0

Author: tetrate-ci

liaison/v1alpha1/resources.proto

@@ -35,6 +35,6 @@ message SetResourceRequest {
 
 // Request to get an uploaded resource.
 message GetResourceRequest {
-  // The resource key.
+  // The resource key. Key = TypeURL:FullName. Stringified as [Key](TypeURL:FullName).
   string key = 1;
 }