[transactions] admin_api interface to abort transaction

[VadimPlh]

Problem

If user set big timeout for transaction only way to abort it by hand is restarting redpanda, because timeouts locates in RAM.

Solution

Add interface to admin_api to abort transaction
There are 2 steps

1. Each transaction can be identify by model::producer_identity + model::tx_seq. So we need to provide interface to get current transaction from transactional manager for partition
2. Interface to abort transaction for partition