Feature Request: `skaffold inspect` commands for deployers and builders

In Cloud Code we add metadata to our skaffold metrics that includes information about which builders are used. Currently, we parse the output of `skaffold diagnose` to figure out which builders are being used. However, this method is brittle and depends on the structure of the output of `skaffold diagnose`, which could potentially change. Instead of relying on parsing the output of `skaffold diagnose` it would be great if there was a `skaffold inspect` command that could return the list of builders that will be used with a specific skafold config. Similarly, it would be great to have a command for identifying deployers, which we want to start reporting metadata for.

#### Proposed Commands

`skaffold inspect deployers`

`skaffold inspect builders`