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/castore/src/blobservice/from_addr.rs

@@ -27,7 +27,7 @@ pub async fn from_addr(
     })?
     .0;
     let blob_service = blob_service_config
-        .build("anonymous", &CompositionContext::blank())
+        .build("anonymous", &CompositionContext::blank(&REG))
         .await?;
 
     Ok(blob_service)