feat(tvix/composition): allow urls as anonymous stores

This allows specifying an url in place of a named reference to another
composition entry, if the castore crate has been compiled with the
 xp-store-composition feature.

Example: `--directory-service-addr cache://?near=memory://&far=memory://`

This would be equivalent to the instantiation via toml file:

```toml
[memory1]
type = "memory"

[memory2]
type = "memory"

[default]
type = "cache"
near = "memory1"
far = "memory2"
```

Note that each anonymous url causes a distinct instance to be created.

Change-Id: Iee5a07a94b063b5e767c704d9cad0114fa843164
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12146
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/store/src/utils.rs

@@ -188,7 +188,7 @@ pub async fn construct_services_from_configs(
     ),
     Box<dyn std::error::Error + Send + Sync>,
 > {
-    let mut comp = Composition::default();
+    let mut comp = Composition::new(&REG);
 
     comp.extend(configs.blobservices);
     comp.extend(configs.directoryservices);