Draft: KEP-2: Support for multiple keys

This KEP is meant as an alternative proposal to KEP-1, and at most one of them should be accepted.

KEP-2 has a larger scope than KEP-1, and partially reimagines the way KOO stores, manages, and serves keys. This means that implementing this KEP would be more effort than implementing KEP-1. However, it aims to set us up not just for the transition between v4 and v6, but also to post-quantum keys, and generally provides a more flexible base. (Nevertheless, policy decisions such as limitations on the number of keys may still be agreed upon, if desirable.)