Add capability to specify cache directory for registry

[nonik0]

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 fetches 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 fetches 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"