Implement absolute filesystem path finding for individual files contributing to authz schema #533

Created from the work on the ADR defined here: #519

Currently the Authz Schema discovery mechanism has two ways to discover and load yaml files with authz schemas mappings, first one is through the package entry point authz.schema (defined in ADR-0019) and through django setting OPENEDX_AUTHZ_SCHEMA_DIRECTORIES

We need to add a third way for the discovery to find the tutor patch (openedx-authz-schema) contributions for Site Operators

In order to implement this, we are introducing a new django setting which will read from absolute filesystem paths (as described in the ADR), also the loading must be able to load multi documents yaml files (right now it expects a single document per file) due to the tutor patch rendering every contribution to the same file”

This story is complement of this other one (the tutor plugin + patch implementation itself): #461