refactor(tvix/store): move construct_services helper here

This takes three URLs, and constructs Arc'ed
{Blob,Directory,PathInfo}Service, allowing to remove some of the
boilerplate.

Change-Id: I40e7c2b551442ef2acdc543dfc87ab97e7c742bb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10484
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

tvix/store/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod nar;
 pub mod pathinfoservice;
 pub mod proto;
+pub mod utils;
 
 #[cfg(test)]
 mod tests;