gateway: product service-resolver that will call product-service client

[pooranjoyb]

[saurabhhhcodes]

Hi @pooranjoyb, I’d like to work on this next under NSoC once #42/#29 is reviewed or if you’re okay with parallel gateway work.

Plan:

• add a ProductServiceClient resolver layer matching the product-service endpoints
• wire GraphQL product queries/mutations through the gateway without touching order resolver logic
• add focused resolver delegation tests similar to the order resolver PR

Could you please assign this to me if it’s available?

[pooranjoyb]

check comment on PR @saurabhhhcodes