Network Bridge Refactoring (#1535)

* rename protocol ID to network capability

* guide: `ProtocolId` -> `NetworkCapability`

* guide: remove `RegisterEventProducer`

* capabilities and expand on underlying network assumptions

* guide: create network.md types file

* guide: network bridge is aware of network messages

* revert changes in code

* Update roadmap/implementers-guide/src/SUMMARY.md

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* remove references to NetworkCapability

* Update roadmap/implementers-guide/src/types/network.md

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Sergei Shulepov <sergei@parity.io>