consumer-contract-example + contract-lib: move trusted verification from example to lib

[juangirini]

Right now, the consumer example implements some useful logic that could be included by the library for ease of use to those developers that prefer a more trusted solution.

• ensure_authorized_deliverer function and the authorized_deliverers storage value with the associated logic. As this requires storage, keep this optional.
• ensure_authorized function
  - [ ] ensure_active_sub function and the subscription_id storage value with the associated logic. As this requires storage, keep this optional.
• ensure_valid_pulse function

This task is about moving those functions to the library, not invoking them. Whether the devs want to use them or not should be up to them

[juangirini]

Following my other comment, ensure_active_sub is not an important verification, it does not need to be on the library. We can leave it in the example as reference but it does not add much