Provide a way to generate output as part of the ytt processing

When combining ytt to kapp for installing some templates I want to be able to give the end user some information about what was installed. An example would be credentials or URL access like what happens when installing applications with Helm.
When not combined with kapp, it probably doesn't make sense, as the output I would expect from ytt will most likely be only the processed templates.

There needs to be a way to, when these 2 commands are chained, ytt will give this information to kapp so that kapp can print it out to the user.

Start by examining how ytt output is passed to kapp when the two commands are chained. Clarify the format and handoff needed for installation information such as credentials or URLs, then verify that kapp can display it to the end user without changing standalone ytt output.

Written by the indexing model from the issue text.