Add documentation to SubmitSignedTransaction and actually make it work (#4200)

* Add documentation to signed transactions and actually make them work.

* Fix naming and bounds.

* Forgotten import.

* Remove warning.

* Make accounts optional, fix logic.

* Split the method to avoid confusing type error message.

* Move executor tests to integration.

* Add submit transactions tests.

* Make `submit_transaction` tests compile

* Remove a file that was accidently committed

* Add can_sign helper function.

* Fix compilation.

* Add a key to keystore.

* Fix the tests.

* Remove env_logger.

* Fix sending multiple transactions.

* Remove commented code.

* Bring back criterion.

* Remove stray debug log.

* Apply suggestions from code review

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Make sure to initialize block correctly.

* Initialize block for offchain workers.

* Add test for transaction validity.

* Fix tests.

* Review suggestions.

* Remove redundant comment.

* Make sure to use correct block number of authoring.

* Change the runtime API.

* Support both versions.

* Bump spec version, fix RPC test.

Co-authored-by: Hernando Castano <HCastano@users.noreply.github.com>
Co-authored-by: Gavin Wood <github@gavwood.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>