add endpoint for premium resource purchases

[PierreSchwang]

fixes #8

Adds 2 endpoints:

1. retrieve all purchases of a premium resource
2. retrieve purchase info of a premium resource purchase by a specific user (validate if a user bought a resource)

I can't really test the change nor do I know the table names and schemas. And I don't really want to pay >$200 USD just for this lol
The API Keys are hopefully correctly implemented.

TODO:

• Update README (when response data format is finalized)
• Update Queries and Response Model (requires table name and structure)

[jacobsandersen]

Thanks for contributing. The structure looks alright. I'll have to try and look at the schema to figure out what needs to be queried.

NB: I don't think the get all purchases by resource id will be allowed, we will only be allowed to get purchases by (resource id, author id) given author id == owner of provided api key.

[PierreSchwang]

NB: I don't think the get all purchases by resource id will be allowed, we will only be allowed to get purchases by (resource id, author id) given author id == owner of provided api key.

Isn't the author inferred by the resource itself and already validated against the API key owner? Not sure if that needs to be provided additionally, as the owner id is not secret whatsoever.