refactor(tvix/*store): helper for channel creation from url

This moves the repetitive code to parse a URL and create a channel
connected to it into `tvix_castore::channel::from_url`.

Part of b/308

Change-Id: Idd342cd71cad5e78a9b258b38c1b227993e75310
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9707
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/castore/src/lib.rs

@@ -2,6 +2,7 @@ mod digests;
 mod errors;
 
 pub mod blobservice;
+pub mod channel;
 pub mod directoryservice;
 pub mod fixtures;
 pub mod import;