AUT-80: Add API endoint to fetch sanitized signer configuration

[oskirby]

In trying to implement the integration test suite with Github Actions in PR #904 I found it rather awkward to replicate the monitor integration tests due our need to pass the root certificate hash around, and this could be made a lot easier if there was a way to fetch public configuration bits from the autograph service, so I wrote an endpoint to do it.

This also aligns nicely with our need for better config discovery and enumeration endpoints, which has been kicking around in JIRA tasks such as:

• AUT-80: Find a new home for sanitized configs.
• AUT-21: Configurations need some way to be explorable.
• AUT-33: Provide endpoints to expose cert validity and fingerprints.