* Greenfield: Improve store users API
- Adds an endpoint to update store users (before they had to be removed ad re-added)
- Checks for the existance of a user and responds with 404 in that case (fixes#6423)
- Allows retrieval of user by user id or email for add and update (consistent with the other endpoints)
- Improves the API docs for the store users endpoints
* Swagger: Reuse UserIdOrEmail parameter component
* Add details to store user data
* Allow overrides on all methods
* Add non-returning SendHttpRequest methods
* Use SendHttpRequest consistently
* Use file-scoped namespace
* Rates: Set default null value for currencyPair
* Ensure it works with instances which have BTCPAY_ROOTPATH set
