Support to merge two views according to a privacy policy

[RafaelAPB]

BUNGEE can now create a view and receive options to do different things with the view. Given a privacy policy and two different views, allow bungee to merge them.

Requirements:

1. Utility functions for a SATP gateway to encode a privacy policy
2. Add functionality to merge two views
3. Add respective endpoints that receive a pair of serialized views and a privacy policy and merge it according to the policy
4. Update prometheus metrics (number of merged views, counter for each privacy policy applied)

[eduv09]

@RafaelAPB please assign me this one