Support pgo builds

[mholt]

Branching from this discussion: caddyserver/caddy#5588

I haven't found any speedup with pgo-optimized Caddy builds, but others have (see linked issue). It could be useful for xcaddy to expose a way to make a pgo-optimized Caddy build, ideally using multiple profiles merged from our user base.

[Forza-tng]

Cloudflare seems to enjoy PGO:
https://blog.cloudflare.com/reclaiming-cpu-for-free-with-pgo

we’re using ~97 cores fewer than before the release, a ~3.5% reduction. This seems to be inline with the upstream documentation that gives numbers between 2% and 14%.