feat: add simple delegation support

[katamarinaki]

Summary

Following the update of the Voting contract (Simple Delegation), this update adds the on-chain delegation to the VotingAdapter contract, giving the TRP participants the ability to delegate their voting power.

Changelog

• Add assignDelegate and unassignDelegate methods to the IVoting interface;
• remove DELEGATION_CONTRACT_ADDR from VotingAdapter contract;
• use VOTING_CONTRACT_ADDR in delegate method;
• add new functions to the mock Voting contract;
• update tests according to the added functionality.