Linting should be able to show warnings

The `elastic-package lint` command validates a package. Most of the heavy lifting for the actual validation is done by the [package spec](https://github.com/elastic/package-spec). Currently, validation either passes or fails.

It would be nice if the package spec + `elastic-package lint` could support the idea of validation warnings, something in between passing and failing. For example, there might be a field defined in the package spec that is recommended but not required. The absence of such a field in a package would cause `elastic-package lint` to show a warning about it but not fail validation completely.

Start with the `elastic-package lint` command and the linked package spec, tracing how validation results currently distinguish passing from failing. Define how a recommended-but-missing field is reported as a warning without failing validation, and verify that required-field failures still fail while warnings are displayed.

Written by the indexing model from the issue text.