feat(tvix/store): add bigtable pathinfoservice backend

Put behind the "cloud" backend, like in the `tvix-castore` crate.

Change-Id: Ib38d198baf11ab2a4b6dc405121676147c424611
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11362
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI

tvix/Cargo.lock

@@ -4505,8 +4505,10 @@ name = "tvix-store"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-process",
  "async-recursion",
  "async-stream",
+ "bigtable_rs",
  "blake3",
  "bstr",
  "bytes",
@@ -4525,6 +4527,9 @@ dependencies = [
  "reqwest",
  "rstest",
  "rstest_reuse",
+ "serde",
+ "serde_qs",
+ "serde_with",
  "sha2",
  "sled",
  "tempfile",