Allow private keys to be stored and rotated externally.

[j3parker]

External storage (vs. in-memory) will work better for scale-up/scale-down activity. This will enable us to scale more aggressively, including use-cases like AWS Lambda.

It also moves rotation out-of-line of requests, which will avoid CPU+latency spikes.

Fetching the latest private key will still be done on-demand, but there is the option to do it proactively out-of-band. This will allow us to consolidate the fetch in web-server workloads and further hide latency from individual requests.

Web server workloads will be able to proactively fetch a key on boot before being placed into load.

[j3parker]

What's missing:

• Tests
• ECDSA support
• Refactoring the in-memory stuff to be built on top of this

The first of those is mandatory to merge this, the other two I'd be good to do in a follow-up (like immediately after, not "in the future")

Getting this PR up without tests just to validate the approach.