Add storage_broker binary.

Which ought to replace etcd. This patch only adds the binary and adjusts
Dockerfile to include it; subsequent ones will add deploy of helm chart and the
actual replacement.

It is a simple and fast pub-sub message bus. In this patch only safekeeper
message is supported, but others can be easily added.

Compilation now requires protoc to be installed. Installing protobuf-compiler
package is fine for Debian/Ubuntu.

ref
https://github.com/neondatabase/neon/pull/2733
https://github.com/neondatabase/neon/issues/2394

storage_broker/build.rs

@@ -0,0 +1,7 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    // Generate code to deterministic location to make finding it easier.
+    tonic_build::configure()
+        .out_dir("proto/") // put generated code to proto/
+        .compile(&["proto/broker.proto"], &["proto/"])?;
+    Ok(())
+}