[dnf5] Implement new argument "--dump-variables"

[jrohel]

If dnf5 is run with this argument, variable values are added to standard output.

The old DNF4 supports this argument only with the config-manager plugin. If the user wants to know the values of the variables with which a certain command will run, he must first use config-manager and then run the desired command with the same settings (same environment and configuration files) and hope that nothing changed in between (e.g. thanks to a plugin).

In other words, this PR implements part of the functionality of the dnf4 config-manager plugin, but for general use.

Related to: #405

[m-blaha]

Just an idea - would it make sense to implement this as a new command (e.g. dnf5 debug)? It could present to the user more information - variables, all applied config options (since configuration is spread among multiple files, this might be interesting), versions of libraries in use (libdnf5, libsolv, librpm, librepo...) and so on.

Copr plugin has the same subcommand used for similar purpose.

[jrohel]

Rebased to resolve conflicts.

[ppisar]

There is missing a documentation in doc/dnf5.8.rst.

[ppisar]

I somehow find it quite obtrusive that "./dnf5/dnf5 --dump-variables" does not work also alone:

$ ./dnf5/dnf5 --dump-variables
Missing command. Add "--help" for more information about the arguments.

But maybe that's on purpose.

[jrohel]

@ppisar
I intended that --dump-variables would print out what variables the given command was running with.
I am considering whether to modify the behavior of dnf5.

Edit:
I added a commit that allows running dnf5 with the --dump-variables argument without the command.

[ppisar]

Documentation and standalone invocation works now.