feat(tvix/store): build rust proto files

This uses prost-build to build the proto files.

Change-Id: Ia99fcfa4a19c741683cf28a45202e1d9211f9131
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7286
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: Adam Joseph <adam@westernsemico.com>

tvix/store/src/lib.rs

@@ -0,0 +1,3 @@
+pub mod protos {
+    include!(concat!(env!("OUT_DIR"), "/tvix.store.v1.rs"));
+}