[Feature] Allow Unknown Options from all sources

For the command line parser, there is the [`allow_unregistered()` method](https://www.boost.org/doc/libs/1_70_0/doc/html/program_options/howto.html#id-1.3.32.6.9) to not throw an exception whenever an option is not recognized.

However, this is missing from the configuration file and environment parsers.

It is possible to do work around this for both [configuration files](https://stackoverflow.com/questions/23664503/parsing-unregistered-options-for-config-files-in-boost-program-options) and [environment](https://stackoverflow.com/questions/53285248/boost-program-options-allow-undeclared-from-environment), but I am of the opinion this should be handled in similar fashion to the command line parsers!

Start with the documented allow_unregistered() behavior for command-line parsing, then compare how the configuration-file and environment parsers handle unknown options. Review the linked configuration and environment workarounds before deciding how the existing parsers should behave. Done means unknown options are handled consistently across all three sources without exceptions.

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