Add an endpoint that will output Athens configuration

**Is your feature request related to a problem? Please describe.**

Athens has many configuration values and it's easy to get them mixed up. We are taking several measures to make it easier to make sure you don't configure it with anything obviously bad.

**Describe the solution you'd like**

Along with [other strategies](https://github.com/gomods/athens/pull/1492) to make this better, it would be helpful to opt-in to an HTTP endpoint that an operator can `curl` and that outputs the current configuration. That would be helpful to help with debugging.

**Describe alternatives you've considered**

We could have Athens log everything on startup.

**Additional context**

N/A

The issue names no files or tests. Start by locating Athens's HTTP routing and configuration initialization, then determine how an operator opt-ins to an endpoint that outputs the current configuration. Done means the endpoint is available only when opted in and its output reflects the running configuration; add coverage for that behavior if the existing test structure supports it.

Written by the indexing model from the issue text.