feat: add `at` query param for staking-payouts (#1388)

* feat: add `at` query param for staking-payouts

* add docs

* rewrite derive to fit historic data

* hack for tests

* add new mock data

* export mock data

* add single new test

* fix test

* update test to reflect validator and nominator

* add extract axposure test

* extractExposure for nom and val

* extractTotalValidatorRewardPoints

* error test

* another error test

* deriveNominatedExposures

* replace original file

* finalize tests