Implement "Resource Prototypes"

[vito]

Summary

This issue is for implementing concourse/rfcs#38 as described, using the Prototypes protocol prescribed in concourse/rfcs#37.

Key points:

• Prototypes are configured under prototypes: in the pipeline, resource types are under resource_types: - this is how we know which protocol/interface to use.
• Resources implemented by prototypes support creating multiple versions from put steps.
• Resources implemented by prototypes have an optional get after put - the user must configure get: and put: on the same step.
• Versions produced by prototypes must be able to be deleted - it may be a good idea to implement Record version history as a tree #4871 in order to support this more easily.
• A new delete step is added, supported by prototypes but not resource types.

Context

• RFC: RFC: Resource Prototypes rfcs#38, RFC: Prototypes rfcs#37