Add open-payments-specifications as submodule

[sanducb]

Context

Open Payments specs now live in a separate repository and should be referenced instead of being manually copied in the openapi-definitions folder. Even though we are not using the specs for generating types, we should reference the spec version that the SDK is currently implementing.

To do

Delete openapi-definitions directory and add open-payments-specifications as a submodule referencing the correct specs version used in the SDK