Reduce Reliance on Global Cache

[yhakbar]

We rely on global variables for shared cache state, which is an anti-pattern we should refactor out of the codebase.

This can cause issues with integration testing, and can increase the memory footprint of Terragrunt.

Develop a new mechanism for explicitly passing around that cache through a cache manager so that we can be more thoughtful with how we're caching content throughout an invocation of Terragrunt. Refactor caching throughout the codebase to leverage it.

[yhakbar]

An interesting observation made was that we generally are looking for caching to support memoization of our function calls.

Leveraging a OnceFunc with something to old the functions for re-use might provide a good pattern for a large scale refactor.