[epic] ClusterExtension parameters passed through to templating engine

[ncdc]

Define a way for the content author to specify what parameters are user-facing, along with a way to plumb them through from the ClusterExtension spec the templating engine

Example use case: I publish a bundle for my Widget app, and the only knobs I want users to be able to tweak are replica count and logging verbosity.

This either needs to accommodate v0 subscription config settings (e.g. env vars, node selector, etc.), or we need a different mechanism for that.

[joelanford]

See #612

In order to keep watch namespace configuration out of the Extension API as a first class field, I think we should define our own schema/values that account for watch namespaces and subscription config, and put that in scope for the initial registry+v1 bundle support in Extension+App

[tmshort]

This should also consider proxy information that is determine dynamically via environment.

[joelanford]

I implemented a proof of concept as part of #1418. We should revisit that PR when we have time to pick this issue up.