feat(router): consider public and private directives for cache control

[df-wg]

Motivation and Context

Previous efforts to add complex cache control policies didn't address the public/private directives which are documented here. This PR adds them, and refactors the cache control code to make it clearer and more maintainable.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.
• I have read the Contributors Guide.