[v2] Add workspaceTemplate to Stack API

[blampe]

This adds a workspaceTemplate field to the Stack API. Also removes resources and image from the API, since those can now be specified via workspaceTemplate and it's confusing for them to be overridable in two places. We can re-add these as a convenience later when we've thought through precedence and validation for them.

Fixes #666

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 10 lines in your changes missing coverage. Please review.

Please upload report for BASE (v2@7fd37bb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...tor/internal/controller/pulumi/stack_controller.go	66.66%	5 Missing and 5 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v2 #669 +/- ## ===================================== Coverage ? 20.66% ===================================== Files ? 25 Lines ? 3769 Branches ? 0 ===================================== Hits ? 779 Misses ? 2867 Partials ? 123 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.