transaction-pool: drop unpropagable txs if local node cant author blocks (#8048)

* transaction-pool: drop unpropagable txs if local node cant author blocks

* fix test compilation

* transaction-pool: remove unnecessary static bound on CanAuthor

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>

* rpc-api: add translation for PoolError::Unactionable

* transaction-pool: add test for rejecting unactionable transactions

* basic-authorship: fix doc test

* transaction-pool: fix benchmark compilation

* transaction-pool: rename CanAuthor to IsValidator

* transaction-pool: nit in error message

Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>