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

[jprendes]

Thanks for working on this! :-)

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

Something like:

pub struct Entrypoint<'a> {
    // The entrypoint function in the wasm module.
    // Usualy "_start".
    pub func: String,

    // A name that can be used to identify the module.
    // Usualy the name of the wasm file.
    pub name: Option<String>,

    // The raw arg0 provived in the OCI spec.
    pub arg0: Option<&'a Path>,

    // The source for the WASI module / components
    pub srouce: WasiSource<'a>,
}

// The source for a WASI module / components.
pub enum WasiSource<'a> {
    // The WASI module is a file in the file system.
    // This case is only valid with WASI preview1 modules.
    File(PathBuf),

    // The WASI module / component is provided in the OCI spec.
    // For a WASI preview 1 module this is usually a single element array.
    // For a WASI preview 2 component this is an array of one or more
    // elements, where each element is a component.
    // Runtimes can additionally provide a list of layer types they support,
    // and they will be included in this array, e.g., a `toml` file with the
    // runtime configuration.
    Oci(&'a [WasmLayer]),    
}

[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

[jprendes]

Awesome! Thanks @jsturtevant !
Just two minor nits, and I'm happy for this to be merged :-)

[Mossaka]

Thanks, lgtm!