[PROPOSAL] change mapping_file config name to mappings_config_key

[gadelkareem]

The mapping_file config is misleading because it internally look for the config key in the application. Moving the config into another path will cause the mappings to break while no instructions to load the mappings config dynamically.

Package version, Laravel version

5.6

Steps to reproduce the behaviour

Moving the config file mappings.php into another path will cause the mappings to break while no instructions to load the mappings config dynamically.