feat(tvix/store): add import::import_path

This imports the contents at a given Path into the tvix store.

It doesn't register the contents at a Path in the store itself, that's up
to the PathInfoService.

Change-Id: I2c493532d65b90f199ddb7dfc90249f5c2957dee
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8159
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/store/Cargo.toml

@@ -24,6 +24,7 @@ tokio = { version = "1.23.0", features = ["rt-multi-thread"] }
 tonic = "0.8.2"
 tracing = "0.1.37"
 tracing-subscriber = { version = "0.3.16", features = ["json"] }
+walkdir = "2.3.2"
 
 [dependencies.tonic-reflection]
 optional = true