Add ability to load OCI with pre-fetch artifacts

[jsturtevant]

Background

This issue in runwasi containerd/runwasi#108 and the approach outlined in https://docs.google.com/document/d/11shgC3l6gplBjWF1VJCWvN_9do51otscAm0hBDGSSAc. Also the changes proposed in #1857

Proposed changes

In kubernetes based environment, the OCI artifacts might be already pre-pulled but the OCI client requires pulling them:

spin/crates/oci/src/loader.rs

Lines 25 to 29 in 362c75d

	pub async fn load_app(&self, client: &mut Client, reference: &str) -> Result<LockedApp> {
	// Fetch app
	client.pull(reference).await.with_context(|| {
	format!("cannot pull Spin application from registry reference {reference:?}")
	})?;

It would be nice to have a function similiar to let app = spin_loader::from_file(PathBuf::from("/spin.toml"), Some(working_dir.clone())).await.context("unable to find app file")?;

but for spin.json that OCI artifacts use.

I've prototyped this working with this change used in deislabs/containerd-wasm-shims#164. But there might be better ways to approach it.

[vdice]

Thanks for creating this @jsturtevant; it shouldn't be a problem to refactor as suggested and provide an additional method for loading an OCI-based Spin app from an already-fetched config (no pull involved).

[itowlson]

@jsturtevant Looking at your diff I can't imagine any reason we'd do it differently (though I might suggest naming the new function something like load_pulled_app or load_from_cache) - would you like to send a PR, or would you rather we implemented it?