refactor(tvix): move castore into tvix-castore crate

This splits the pure content-addressed layers from tvix-store into a
`castore` crate, and only leaves PathInfo related things, as well as the
CLI entrypoint in the tvix-store crate.

Notable changes:
 - `fixtures` and `utils` had to be moved out of the `test` cfg, so they
   can be imported from tvix-store.
 - Some ad-hoc fixtures in the test were moved to proper fixtures in the
   same step.
 - The protos are now created by a (more static) recipe in the protos/
   directory.

The (now two) golang targets are commented out, as it's not possible to
update them properly in the same CL. This will be done by a followup CL
once this is merged (and whitby deployed)

Bug: https://b.tvl.fyi/issues/301

Change-Id: I8d675d4bf1fb697eb7d479747c1b1e3635718107
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9370
Reviewed-by: tazjin <tazjin@tvl.su>
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/Cargo.lock

@@ -2730,6 +2730,37 @@ version = "0.2.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3528ecfd12c466c6f163363caf2d02a71161dd5e1cc6ae7b34207ea2d42d81ed"
 
+[[package]]
+name = "tvix-castore"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "async-stream",
+ "blake3",
+ "bytes",
+ "data-encoding",
+ "futures",
+ "lazy_static",
+ "pin-project-lite",
+ "prost",
+ "prost-build",
+ "sled",
+ "tempfile",
+ "test-case",
+ "thiserror",
+ "tokio",
+ "tokio-stream",
+ "tokio-util",
+ "tonic",
+ "tonic-build",
+ "tonic-mock",
+ "tonic-reflection",
+ "tower",
+ "tracing",
+ "url",
+ "walkdir",
+]
+
 [[package]]
 name = "tvix-cli"
 version = "0.1.0"
@@ -2742,6 +2773,7 @@ dependencies = [
  "thiserror",
  "tokio",
  "tracing",
+ "tvix-castore",
  "tvix-eval",
  "tvix-store",
  "wu-manber",
@@ -2832,6 +2864,7 @@ dependencies = [
  "tower",
  "tracing",
  "tracing-subscriber",
+ "tvix-castore",
  "url",
  "walkdir",
 ]