Allow to compute a RabbitMQ user password hash

Summary:
RabbitMQ management API expects user password to be hashed
according a specific algorithm.

This RabbitMQ execution module implements that algorithm.

Reference: https://rabbitmq.com/passwords.html#computing-password-hash

Test Plan: Unit test provided with the example specified in RabbitMQ documentation

Reviewers: Sandlayth, DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D2717