Modify Inventory holdings to MFHD record mapping rules file

Purpose:
Libraries depending on their profiles might have a different needs for inventory holdings to MFHD mappings. This user story is limited to the backend functionality that would allow to modify the standard mapping rules file. The calls made by API might later be used by command line or GUI.

Scenario:

• Given: A standard mapping rules file
• When: Working on specific library needs
• Then: a mapping for a specific field can be added, removed or updated

Acceptance criteria:

• MFHD record is generated according to the modified rules
• Mapping rules file is tenant specific