proxy: support dynamic port number

[lelemka0]

This PR delays address parsing in some case to make dynamic port numbers work properly.
If dial addresses contains { and }, address parsing in Upstream.provision will be skipped then done later in Handler.dialPeers after all placeholders are replaced, and active health checks will be ignored.

Example Caddyfile:

layer4 {
	:2000 {
		@test tls sni_regexp ^([0-9]{1})\.test$
		route @test {
			proxy {
				upstream 127.0.0.1:800{tls.regexp.1}
			}
		}
	}
}