add support for changes to computed arrays and maps

[kyleadams-nflx]

Currently, if the diff includes a map or array that is computed, the parser will fail. This is because it assumes that the only way a map or array could be changed at a top level, is if it gets nulled out. Otherwise any changes would appear in the body of the array or map. So this works:

      - services           = [
          - "appengine.googleapis.com",
          - "audit.googleapis.com",
        ] -> null

However if the map or array is set to computed, such that its value is (known after apply), then the format of the plan is similar to the whole map or array being replaced, but doesn't end in "null", so the parsing logic doesn't know what to do.

      ~ services           = [
          - "appengine.googleapis.com",
          - "audit.googleapis.com",
        ] -> (known after apply)

This PR adds the (known after apply) as an alternative expected ending to an array or map field. Tests have been included to validate the new behavior.