Handle default toml file in TomlGatewayConfigParser differently

## Brief feature description

Currently `TomlGatewayConfigParser::parse()` has as a default argument the default iceoryx config file. This leads to failing tests when the user has a non empty gateway configuration. Furthermore, the config file path is stored in the toml parser and not the `platform_settings.hpp` file.
This should be stored in a platform dependent settings file in posh - not hoofs!

## Detailed information

1. Remove default argument from parse.
2. Adjust unit test: `ParseWithoutParameterTakeDefaultPathReturn`
3. Create `platform` directory in posh, similar to hoofs which stores the posh specific platform settings
4. Add default toml gateway file path to platform settings in posh