[concourse] Support validation and completions inside `params` of `resource_type`

[kdvolder]

Example:

resource_types:

- name: cogito
  type: registry-image
  check_every: 24h
  params:
    <*>
  source:
    repository: ((docker-registry))/cogito

Invoking completions where the cursor <*> is should suggest attributes as in the params of a get step of a registry_image resource. These are documented here: https://github.com/concourse/registry-image-resource#in-fetch-the-images-rootfs-and-metadata

At the moment it only has one valid attribute called format

Similarly any attributes below the params should be validated (i.e. at the moment the only valid attribute is format.

Related discussion: concourse/docs#273