Support external shell interface modules

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.

Start in Rex::Interface::Shell by examining the %SHELL_PROVIDER map and the remote-shell discovery path. Compare the behavior for the external Rex::Interface::Shell::Ilo and Rex::Interface::Shell::Idrac4 modules, then establish whether discovery should skip unavailable modules or replace the provider mapping; done means available providers load without aborting discovery and no-match cases are logged.