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/errors.rs

@@ -34,6 +34,12 @@ impl From<Error> for Status {
     }
 }
 
+impl From<crate::channel::Error> for Error {
+    fn from(value: crate::channel::Error) -> Self {
+        Self::StorageError(value.to_string())
+    }
+}
+
 // TODO: this should probably go somewhere else?
 impl From<Error> for std::io::Error {
     fn from(value: Error) -> Self {