feat(tvix/nar-bridge): init

This provides a Nix HTTP Binary Cache interface in front of a tvix-store
that's reachable via gRPC.

TODOs:

 - remove import command, move serve up to toplevel. We have nix-copy-
   closure and tvix-store commands.
 - loop into CI. We should be able to fetch the protos as a third-party
   dependency.
 - Check if we can test nar-bridge slightly easier in an integration
   test.
 - Ensure we support connecting to unix sockets and grpc+http at least,
   using the same syntax as tvix-store.
 - Don't buffer the entire blob when rendering NAR

Co-Authored-By: Connor Brewster <cbrewster@hey.com>
Co-Authored-By: Márton Boros <martonboros@gmail.com>
Co-Authored-By: Vo Minh Thu <noteed@gmail.com>

Change-Id: I6064474e49dfe78cea67676957462d9f28658d4a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9339
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/nar-bridge/cmd/nar_bridge/main.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"os"
+
+	"github.com/alecthomas/kong"
+)
+
+//nolint:gochecknoglobals
+var cli struct {
+	// TODO: make log level configurable
+	Import ImportCmd `kong:"cmd,name='import',help='Import a local NAR file into a tvix-store'"`
+	Serve  ServeCmd  `kong:"cmd,name='serve',help='Expose a tvix-store RPC interface as NAR/NARInfo'"`
+}
+
+func main() {
+	parser, err := kong.New(&cli)
+	if err != nil {
+		panic(err)
+	}
+
+	ctx, err := parser.Parse(os.Args[1:])
+	if err != nil {
+		panic(err)
+	}
+	// Call the Run() method of the selected parsed command.
+	err = ctx.Run()
+
+	ctx.FatalIfErrorf(err)
+}