Add capability to specify cache directory for registry

I've found that a common footgun for myself during dev workflows is that I "poison" my local wkg cache with locally built packages while I do dev work. Then these dev packages find their way into later `wkg fetch`es when I just want to be pulling the latest from OCI.

Adding the ability to specify a cache directory for a registry could help here. I can specify a separate cache for my local registry where my locally versioned dev packages are built/published and dev scripts and such can specifically point `wkg fetch`es to this cache. Other workflows continue to use `wkg fetch` with the default cache, which will only ever contain packages from OCI that have been published and released.

e.g.:

```
default_registry = "acme.registry.com"

[registry."acme.registry.com".oci]
auth = { username = "open", password = "sesame" }
protocol = "https"

[registry."acme.registry.com".local]
root = "/a/path"
cache = "/not/default/wkg-cache/loc"
```