Should we register a default sanitizer that updates user agent to include proxy?

Essentially, we want a unified way to ensure that proxy-recorded traffic is readily visible.

@weshaggard @benbp

The proxy already has a set of default sanitizers (`authorization` header, body keys with secrets), so one additional one that _updates_ the `User-Agent` value with a `-proxy` suffix or the like is totally doable.

This _would_ be a breaking change across the board, but I'm not certain that should prevent us.