Let users set claimer for themselves

[LHerskind]

The issue was first discussed in #6 (comment)

The setClaimer functionality is currently only available to the emission manager, but giving a user the ability to set the claimer for himself could be a useful feature that allows better separation of access control for holders that want to have their Aave funds in cold storage but still claim using a hot wallet.

Suggestion:
Add a separate setClaimer() function that allow any user to set a claimer for themselves.

function setClaimer(address caller) external override {
  _authorizedClaimers[msg.sender] = caller;
  emit ClaimerSet(msg.sender, caller);
}