Re-introduce purchase endpoints

[msacrea]

By the end of 2024 no implementation of purchase functionality via the ca-pension API is known to us. At the same time the purchase endpoints have never been reviewed and agreed upon by the community. To avoid confusion we remove the purchase endpoints from the API.

When the need arises they can be properly reintroduced. As a starting point the initial definitions could be used. They are attached to this issue in a file: purchase-definitions.txt. Content: Descriptions (DE and EN), Paths & Components.