[AUDIT] Feature: CrossChain PP Base & Everclear CrossChain PP

[Zitzak]

Audit Scope

This PR introduces two new contracts to the dev branch for which an Audit is requested. Those two contracts are as following:

1. PP_CrossChainBase_v1: An abstract contract implementing the base functionalities for a crosschain payment processor.
2. PP_Everclear_CrossChain_v1: An implementation contract integrated with the Everclear bridge, which adds implementation specific functionalities to the PP_CrossChainBase.

Both contracts have unit tests