Add part to ingest Relationships between Data Entities

[RamanDamayeu]

We need to prepare spec to ingest relationship between data entities:

1. We assume that relationship could be only between 2 data entities (and self-reference included)
2. We prepare only 2 types of relationship at the moment: between relations (tables/files/view/etc.) and between graph nodes
3. We assume that there would be this list of attributes for different types:
   3.1 Between relations: 1) cardinality (One-to-Zero-One-or-More, One-to-One-or-More, One-to-Zero-or-One, One-to-Exactly-1); 2) Identifying/Non-identifying; 3) ODDRNs of data entities for beginning/end; 4) ODDRNs of data set fields for beginning/end (there could be composite foreign keys);
   3.2 Between graph nodes: 1) Relationship Types (name); 2) Direction; 3) List of Attributes (key-value); 4) ODDRNs of data entities for beginning/end;