Add support for controlling Pex CLI via env var or arg.

[jsirois]

Currently the Pex CLI(s) only support command line arg control. The env var control is only formally supported and documented via --help-variables for runtime control over PEX files. Like many options systems, it would be good if the Pex CLIs could be controlled by either env var or command line arg for every command line arg they currently accept. It probably also makes sense to support toml config since toml is now vendored to support PEP-517 / PEP-518. The latter would provide, amongst other things, a nice way to seal in modern defaults for Pex CLI options like --resolver-version pip-2020-resolver and --pip-version ....