Add Tx Indexer Extension (#1143)

* Remove rejected from controller interface

* Update mocks and remove unused mock_auth from mocks.mockgen.txt

* Refactor tx indexing into indexer extension

* Close txDB in morpheusvm on shutdown

* Update name of unused result param to _

* fix lint

* Update license year

* Address nits

* move action handler to its own accepted subscriber

* Add interface check for noopTxIndexer

* Fix typo

* fix unused receivers

* fix linting

* invoke combined subscriber to fix integration test

* move action handler prior to tx indexer to fix pre-existing race condition

* Add TODO to integration test flagging flaky test

* Noop -> NoOp

* Address comments

* Export NoOpTxIndexer

* rename combinedAcceptedSubscriber AcceptedSubscribers