Support subscription to pending txs

Seems like subscription to pending txs doesnt work.
Could be nice if it can be added or if we can add general subscriptions through the configuration file (similar to how methods are added today).

Start by tracing the existing transaction-subscription handling and the configuration-file mechanism used to add methods. Determine whether pending transactions are absent from the current subscription path or whether configuration support is needed; done means pending transaction subscriptions work and are covered by the relevant tests.

Written by the indexing model from the issue text.