Add Gitops support for multiple stacks

[BSick7]

Overview

Currently, IaC files must be located in .nullstone/ when configuring gitops connections.
To support monorepos, it would be helpful if a developer could configure multiple stacks within a single repo.

Details

Allow developers to specify the following tree structure:

.nullstone/
  stacks/
    stack1/
      config.yml
    stack2/
      config.yml

If both stack1 and stack2 have an enabled gitops connection, the IaC files are pulled from each of their respective directories instead of pulling from the root .nullstone/