Feat: Add PoW for messages (#511)

* add pow and nonce to the message

* add pow plugin

* fix pow tests

* fix linter warnings

* fix typo

* fix panic when messagelayer is disabled

* improve logging

* fix typo

* make the pow check a byte filter

* decrease the test PoW difficulty even further

* expose the pow filter and use it in the pow plugin

* Apply suggestions from code review

* make length of nonce a const

Co-authored-by: Luca Moser <moser.luca@gmail.com>