httpcaddyfile: Implement log sampling config

[nikonhub]

Closes #6680

Support log samplig in Caddyfile

{
	auto_https off
	log {
		sampling {
			interval 100
			first 100
			thereafter 100
		}
	}
}

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

Please also add an adapt tests in https://github.com/caddyserver/caddy/tree/master/caddytest/integration/caddyfile_adapt, both for the log global option and for the log directive (within a site block).

You can ignore the goreleaser-check failure for now. Our fault, it should skip that check on PRs from forks.

[nikonhub]

I've added 2 caddy file test

[mholt]

Very cool, thanks for the contribution!