Hierarchical Spacelift Config

Currently, stacks require the .spacelift/config.yml to be present at the root of a multi-root repository.

I would like to not have to trigger other stacks when the .spacelift/config.yml changes for a specific stack, but there is no way to tell what changes were made to the file (we can only tell that the file itself was changed). The closest thing that we currently have is repository_runtime_config_changed, which is still undocumented but nevertheless a feature of approval policies.

I would propse that one of two things happen (or both):

1. repository_runtime_config_changed is implemented as a field in the data passed to push policies

2. .spacelift/config.yml can be specified in an hierarchical manner, meaning a specific project root could have its own project-root/.spacelift/config.yml (which could furthermore be merged with the root .spacelift/config.yml so that defaults could be specified at the repo-level, as an example).