feat: settings during code generation

[Apollorion]

This PR allows us to pass specific settings, with defaults, to our Jinja templates.

Added:

generator:
  spacelift:
    manage_state: bool # Instructs the spacelift stacks to manage state or not
  github:
    custom_app: bool # Instructs Jinja to add a github_enterprise block or not

github_enterprise in a spacelift_stack is meant to be used when setting up a custom github application. Since there is no way for us to know beforehand how they set this up, adding it as a flag to the generator config.