Move reference handling to a separate class

Problem/Motivation

References between external Bitrix24 entities and corresponding Drupal entities are now handled in the same service that is responsible for REST requests to the Bitrix24 API. This is architecturally incorrect.

Steps to reproduce

Proposed resolution

Move the processing of bitrix24 references to a separate service.

Remaining tasks

Move methods
Create method for find a "back" reference (internal entity id by B24 entity id)
Delete user references when deleting user.

User interface changes

No

API changes

Methods for working with references (getReference, deleteReference, getHash, updateHash) will be moved from Drupal\b24\Service\RestManager to a separate service.

Data model changes