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/Cargo.lock

@@ -2809,6 +2809,7 @@ dependencies = [
  "tonic-reflection",
  "tracing",
  "tracing-subscriber",
+ "walkdir",
 ]
 
 [[package]]