Proposal to have the DiscoveryService to do bidirectional sync

[venilnoronha]

Currently, the DiscoveryService allows only unidirectional sync. One of the challenges with this approach is that most agents implementing this interface need to be publicly available.

https://github.com/vmware/hamlet/blob/9a69c19c362ddf4793101d49ade6797c703a8dc5/api/resourcediscovery/v1alpha1/resource_discovery.proto#L88-L95

The proposal is to change the interface in a way that service discovery works seamlessly even when only one of the agents is publicly available while the rest may not.

-  rpc EstablishStream (stream StreamRequest) returns (stream StreamResponse);
+  rpc EstablishPublishStream (stream StreamRequest) returns (stream StreamResponse);
+  rpc EstablishSubscribeStream (stream StreamResponse) returns (stream StreamRequest);

/cc @sergiopozoh

[sergiopozoh]

Pending #25