guest: debian: Implement storage_balloon_agent

Add storage_balloon_agent daemon in Debian and its client in
the TerminalApp for enabling storage ballooning.
This feature is hidden behind a feature flag 'terminalStorageBalloon'.

Since we still use non-sparse disks, the balloon shouldn't affect
the guest's disk space yet.

Bug: 382174138
Test: Run a VM and check logs

Change-Id: I75d926bb8aa8a02bf635e94e35715e5aa23c8090
diff --git a/libs/debian_service/proto/DebianService.proto b/libs/debian_service/proto/DebianService.proto
index 43955fa..e52b28a 100644
--- a/libs/debian_service/proto/DebianService.proto
+++ b/libs/debian_service/proto/DebianService.proto
@@ -25,6 +25,7 @@
   rpc ReportVmActivePorts (ReportVmActivePortsRequest) returns (ReportVmActivePortsResponse) {}
   rpc OpenForwardingRequestQueue (QueueOpeningRequest) returns (stream ForwardingRequestItem) {}
   rpc OpenShutdownRequestQueue (ShutdownQueueOpeningRequest) returns (stream ShutdownRequestItem) {}
+  rpc OpenStorageBalloonRequestQueue (StorageBalloonQueueOpeningRequest) returns (stream StorageBalloonRequestItem) {}
 }
 
 message QueueOpeningRequest {
@@ -52,3 +53,9 @@
 message ShutdownQueueOpeningRequest {}
 
 message ShutdownRequestItem {}
+
+message StorageBalloonQueueOpeningRequest {}
+
+message StorageBalloonRequestItem {
+  uint64 available_bytes = 1;
+}