Add Config file for downloader and checker

[JanHoefelmeyer]

Allow the configuration of the CSAF-checker and CSAF-downloader to be read via a seperate config file, while maintaining the option for command line parameters.

[JanHoefelmeyer]

The functionality of using a config file for the csaf_downloader was implemented via:
#404 (Support for config files in downloader)

After deciding that the TOML file format offers advantages over the previously used format, the switch to TOML files as config files was made via:
#405 (Use TOML as config file in downloader)

Similarly, a TOML config file option was introduced for the checker via:
#412

Both tools will now search for a TOML file within a few set locations, unless a command line parameter explicitly points towards a location where one should be found. Any TOML file option can be overridden via command line parameters and they can be used to complement each other or used exclusively.

How to use these TOML files is written within their respective tools documentation:

Csaf_downloader: https://github.com/csaf-poc/csaf_distribution/blob/main/docs/csaf_downloader.md

Csaf_checker: https://github.com/csaf-poc/csaf_distribution/blob/main/docs/csaf_checker.md

Thus, this issue should be completely implemented.

[s-l-teichmann]

This is now fully implemented