Support folder level codecov.yml files

[drazisil-codecov]

Use case is for monorepos, or module level component configuration. The idea being they would be merged together in a tree, similar to how instance -> org -> repo is now.

This would allow the Codecov configuration file to be broken more into readable chanks, instead of as a massize single file when many flags or components are designed.

I think of https://www.typescriptlang.org/docs/handbook/project-references.html as a example, though the customers are welcome to provide other examples :)

This is a customer request.