Create an endpoint where the blood alcohol level can be queried

[Tschonti]

• Do some research on how blood alcohol level is calculated.
• Create a GET /users/bac endpoint and a service method to query the necessary data and calculate the value.
• create a DTO for the return value

This endpoint will eventually return the blood alcohol level of the signed-in user. However, right now the DrinkActions are not connected to the users. So you can do the calculation based on all the drinkActions in the database, we'll add the filter for the user later. Also, the gender and weight of the user will probably be needed for the calculation, you can hardcode some parameters for that, but make sure it will be easily extendable once we have this data (after #23 and #24)