Refactor event getting / approval getting code to be more generic

[rkalis]

Right now, we retrieve a bunch of events with different formats, that we read from in our different approvalToAllowance functions. Ideally we'd add an intermediate data structure that normalises the underlying raw blockchain data into an abstract ApprovalEvent (or more generic TokenEvent to also include transfers). This should make it easier to ad support for other chains that work slightly different from most EVM chains (such as LUKSO or Starknet).