Consolidate the RuntimeContext API to use one EntryPoint and expose Source for WASI files

[jsturtevant]

This PR makes a few changes to the the RuntimeContext API regarding OCI:

• use an enum which makes the API clearer when using.
• changes the way the wasm layers are filtered based on https://github.com/containerd/runwasi/pull/147#discussion_r1371838109 
• uses a single entrypoint function instead of 2. I was initially confused when looking at entrypoint vs wasiEntrypoint and thought this might simplify it. Feedback welcome.

[jsturtevant]

The wasmedge error looks like something with k3s, going to re-trigger

NAMESPACE     NAME                                     READY   STATUS              RESTARTS   AGE
kube-system   metrics-server-648b5df564-vfq4j          0/1     Running             0          8s
kube-system   coredns-77ccd57875-zcdxr                 1/1     Running             0          8s
kube-system   local-path-provisioner-957fdf8bc-qdlnl   1/1     Running             0          8s
kube-system   helm-install-traefik-crd-rtw9s           0/1     Completed           0          8s
kube-system   helm-install-traefik-tsb9m               0/1     Error               0          8s
default       wasi-demo-79d9475fd-5bnwj                0/2     ContainerCreating   0          0s
default       wasi-demo-79d9475fd-6tvpx                0/2     ContainerCreating   0          0s
default       wasi-demo-79d9475fd-xl4xf                0/2     ContainerCreating   0          0s

[jsturtevant]

I'm not sure how I feel about WasiLoadingStrategy. I personally would make it part of the entrypoint.

nice! I was struggling with the entrypoint vs loading strategy but hadn't figured it out yet. They shared similar data and logic, I believe your suggestion resolves the dissonance I was feeling. Will give this a go