Support resource_types[].version.digest

[nakamorichi]

What

I want to specify image digest for custom resource types.

Why

It should be possible to guarantee consistency of builds, and using only image tag for custom resource types doesn't guarantee that the same image is always used.

Ref: concourse/concourse#8512

[ChrisJBurns]

We're coming into the scenario now where we'd like to use this functionality as we want to be able to pin our resource_types