We need to add VictoriaMetrics' vmagent to pmm2-client packages (rpm, deb, tar.gz, etc).

• https://victoriametrics.github.io/vmagent.html

• Monorepo: https://github.com/VictoriaMetrics/VictoriaMetrics

It is a third-party code, so it should be built like other third-party code like Prometheus and not like first-party code like other pmm2-client components. Specifically, it should not be a git submodule in https://github.com/Percona-Lab/pmm-submodules - we don't expect to have feature builds of it.