feat: adds example HRC904 contracts + airdrop test coverage + hrc904Proxy test coverage

[simzzz]

Description:

This PR adds example contracts for HRC904 #1012 functionality such as cancelAirdrops, claimAirdrops, airdropTokens, rejectTokens as well as test coverage for airdropTokens

Related issue(s):

Fixes #1109

Notes for reviewer:
As of 20.12 this PR is incomplete, I have added the additional items in the checklist below that need to be done:

Checklist

• Figure out a way to make assertions on the child transaction (my biggest issue currently)
• Fix failing tests where the balance is not correct possibly due to race condition
• Polish code, export util functions, remove redundancy in code, etc.
• Add the correct response codes and message in the assertions