Add prototype for distributing Spin applications using OCI

This commit adds experimental support for distributing Spin applications
using OCI registries.

Specifically, it uses the OCI Artifacts specification
(https://github.com/opencontainers/artifacts) to package and distribute Spin
applications.

This PR implements `spin oci push`, `spin oci pull`, and `spin oci run`
to interact with a supporting container registry - for example:

```bash
$ spin oci push ghcr.io/<username>/my-spin-application:v1
INFO spin_publish::oci::client: Pushed "https://ghcr.io/v2/<username>/my-spin-application/manifests/sha256:9f4e7eebb27c0174fe6654ef5e0f908f1edc8f625324a9f49967ccde44a6516b"
$ spin oci pull ghcr.io/<username>/my-spin-application:v1
INFO spin_publish::oci::client: Pulled ghcr.io/<username>/my-spin-application:v1@sha256:9f4e7eebb27c0174fe6654ef5e0f908f1edc8f625324a9f49967ccde44a6516b
$ spin oci run ghcr.io/<username>/my-spin-application:v1
INFO spin_publish::oci::client: Pulled ghcr.io/<username>/my-spin-application:v1@sha256:9f4e7eebb27c0174fe6654ef5e0f908f1edc8f625324a9f49967ccde44a6516b
```

Following the SIP (#1033), this PR
defines a new `config.mediaType` for a Spin application,
`application/vnd.fermyon.spin.application.v1+config`, and two media
types for the potential content that can be found in such an artifact:
`application/vnd.wasm.content.layer.v1+wasm` for a Wasm module, and
`application/vnd.wasm.content.layer.v1+data` for a static file.
(Note that the media types *can* change in a future iteration of this
experimental work if a community consensus on the media type used to
represent Wasm is reached.)
Following the SIP, this PR distributes the Spin lockfile for a given
application as the OCI configuration object.

This PR also introduces a global cache for layers and
manifests pulled from the registry. This is a content addressable cache,
and its use will be extended in the future for Wasm modules pulled from
HTTP sources.

Currently, `spin oci pull` (or `spin oci run`) will always make at least
an initial request to the registry to fetch the manifest from the
registry. After the manifest is fetched, already pulled layers will not
be pulled again.
In a future commit, the behavior of the initial manifest fetch
regardless of its existence in the cache will be controllable by a flag.

Signed-off-by: Radu Matei <[email protected]>

Author: radu-matei

.gitignore

@@ -1,6 +1,6 @@
 target
 .husky
-cache
+# cache
 node_modules
 ignored-assets
 main.wasm