A flag to install Salt Extensions and other PyPI packages

[max-arnold]

Description of Issue/Question

So, with all these modules being moved out of Salt, it would be good to have а way to bootstrap a Salt Master or Minion with a specific set of extensions and other PyPI packages as well (so when Salt starts, the extensions are already installed). Something like the -p flag for salt-bootstrap.sh, but to install modules from PyPI. And the same for Windows Salt bootstrap script.

The important thing is that the modules should be installed before Salt starts, otherwise some automations like reactors or startup_states can kick in too early.

The module versions should be pinnable

[max-arnold]

A related issue: #1933