Context:
We currently derive product tag (which is now our identifier) from products/engineering ids later in the process and its messy. We want to do that determination up front and save it separately to clean up downstream processes.

We finished adding metered flag to offering table in SWATCH-1956. This card is to add product_tag in offering table after adding logic as defined below.

When an Offering UMB message is ingested we should determine the product tag using role, eng ids(because non-payg may not have role set like Satellite server). If only engid is present then we have to check by parent sku and then child skus to determine the tag. Also if there is a change in role then we should account an extra case in UpstreamProductData > offeringFromUmbData method.

Done:

• We then have to store a new column on the Offering table.
  "Metered" column to differentiate between payg and non-payg.
• Add a new table sku_product_tag for relational mappings.
• The product tag determination can be tested using the  /internal/offerings/{sku}/product_tags endpoint. 
• The changes of this card resolve the bug reported in SWATCH-1840