Add extensible values field to Extension spec

[oceanc80]

In scope:

• Registry+v1 bundles only
• watchNamespaces
• Stretch: subscription.spec.config

[joelanford]

I think we should hold off on making watchNamespaces a first-class field in the Extension API.

Instead, I think we should implement a generic values-style field in the Extension API (similar to Carvel's PackageInstall.spec.values), which we would pass down into the App CR.

For registry+v1 bundles, we will control the YTT template, so we can implement that template to handle a watchNamespaces value if it is defined.

cc @varshaprasad96