Support external shell interface modules

[ferki]

User story

As a Rex maintainer.
I would like to support external shell interface modules,
so I can enable more authors to provide them.

Additional context

Thanks to @alip, there are now external shell modules available on CPAN, like Rex::Interface::Shell::Ilo and Rex::Interface::Shell::Idrac4.

Currently we would need to add them explicitly to %SHELL_PROVIDER map in Rex::Interface::Shell, but even then Rex would bail out when it can't load some of these while trying to discover the remote shell.

Describe the solution you'd like

Ideally, Rex would only try to load shell modules that are available. For bonus points, there might also be a way to make the shell provider mapping obsolete (e.g. by using standardized naming, like "uppercase initial, lowercase rest", or adding heuristics to try some of these variations).

Describe alternatives you've considered

Alternatively a simple error checking might do as a minimal implementation. If it tries to load a shell module but fails, let's log it as a debug message, but continue trying the next one. If no matches found, log a message about it.