Shim fetchStrategy: support arch-agnostic config #574
Description
Currently a Shim's fetchStrategy supports only anonymousHttp and under this configuration a static location is expected, representing the URL to the shim binary location to fetch/download. This means the location is specific to a given architecture. This in turn means the entire Shim resource itself is tied to a particular shim architecture.
It would be preferable to allow specifying architecture-agnostic configuration for where to fetch the shim binary from. Since the actual download logic is currently in the form of the download_shim.sh script, it is able to introspect the arch it is running in (currently, in the form of a Docker image, in which we ship linux/amd64 and linux/arm64 flavors as of writing).
If one surveys the test shims, a common pattern emerges of supplying a GitHub release URL as the location. Extending this, one idea could be to add a new githubRelease fetchStrategy wherein the release URL would be provided (either location or url), e.g. https://github.com/spinframework/containerd-shim-spin/releases/tag/v0.22.0 and an artifact/asset prefix or regex/splat, something like e.g. artifactSplat: containerd-shim-spin-v2-* (continuing the example using containerd-shim-spin).
That last bit just represents a first idea off the top of my head. The general goal here is to uncouple a Shim resource (specifically its shim binary download configuration) from node architecture -- in other words, let the shim-downloader logic figure out which binary suits the architecture it is running on.