Add a global config for Shuttle

Either reopen https://github.com/awslabs/shuttle/pull/218 or create a new one.

I think we'd want to do it slightly differently.

Closed that one because Dylan is no longer around, adding the config is not urgent, and I want to clean up pull requests (make all PRs reviewable and possible to drive to a merge in their current state).

Start by reviewing the closed pull request #218 and the issue discussion to understand the proposed global configuration and why it should differ. Before coding, confirm the intended design and scope with maintainers; done means an agreed approach is implemented in a reviewable pull request that can be driven to merge.

Written by the indexing model from the issue text.