Export Envoy version as string

*Title*: Export Envoy version as string

*Description*:
The `/stats` handler exports `server.version` (and `/stats/prometheus` likewise `envoy_server_version`), but it's a bare number (e.g. `9417675`), with no structure and is not human readable. In contrast, /server_info has a nicely structured `user_agent_build_version` block, but that isn't easily able to be scraped with Prometheus. The plain number is sufficient for spotting outliers (e.g. if not all Envoy instances in a fleet have been updated), but isn't pleasant to read.

It'd be nice if Envoy exported a stat that is a human-readable string (e.g. "1.16.0") on `/stats` and `/stats/prometheus`. This would make it easier to identify the state of outliers in a fleet, and also easier to analyse relative behaviour or performance when rolling out a new Envoy release.