RFC: extract core resource types

[vito]

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[kcmannem]

This is something we'll need to tackle coming soon. Are we ready to go down this approach?

Ignoring implementation, If we construct a stable interface we can build for future runtimes (containerd, k8s, nomad, etc...) as they all have image fetching built in. We can leave the current image_fetcher as a runtime-specific branch in our code and flip it on when using garden.

cc @concourse/runtime

[xtremerui]

How you all feel about we now starting release a concourse binary that only contains registry-image-resource as extra? This would be a chance to trial out the approach in this RFC without affecting existing users. So we could have a better understanding of the impact in the future.