* Have backend.ListStacks return a new StackSummary interface
* Update filestake backend to use new type
* Update httpstate backend to use new type
* Update commands to use new type
* lint
* Address PR feedback
* Lint
This change implements the same preview behavior we have for
cloud stacks, in pkg/backend/httpbe, for local stacks, in
pkg/backend/filebe. This mostly required just refactoring bits
and pieces so that we can share more of the code, although it
does still entail quite a bit of redundancy. In particular, the
apply functions for both backends are now so close to being
unified, but still require enough custom logic that it warrants
keeping them separate (for now...)
This renames the backend packages to more closely align with the
new direction for them. Namely, pkg/backend/cloud becomes
pkg/backend/httpstate and pkg/backend/local becomes
pkg/backend/filestate. This also helps to clarify that these are meant
to be around state management and so the upcoming refactoring required
to split out (e.g.) the display logic (amongst other things) will make
more sense, and we'll need better package names for those too.