Add termination protection and refactor GHA caching

[mdial89f]

Closes #111
Closes #107

This adds cloudformation termination protection for a list of stage names that are commonly used for important environments. This has the effect of requiring a --force flag to be set if destroying a protected stage with serverless (something we never do programatically, so a human would need to set that). In the console, to delete a protected stack, the termination protection must be manually edited to be disabled.
We have a few safeguards in place to prevent deletion of important stages, but this is a huge add since it's protection baked into cloudformation itself.

    stages: # This is a list of common names for important envs that should not be destroyed.  You can remove the stage names your project doesn't use; this list is meant to be inclusive.
      - master
      - val
      - production
      - develop
      - main
      - impl
      - val
      - prod

Obviously a project only needs to protect the stages it cares about (usually master, val, production), but I added all that we've seen to be more safe than sorry.

The github actions caching logic was also refactored. Same end result, just less code and less things to update when adding a new service.

See issues for more detail.