P2P: fix UnknownCapabilityvalidation and maintainig

[AnnaShaleva]

Description

This PR fixes peer discovery mechanism for the nodes with at least one UnknownCapability included into the list of node's capabilities.

Follow the logic implemented in nspcc-dev/neo-go#3778: the node will relay UnknownCapability information through the network even if it's not known to the node itself. @roman-khimov, you may argue with the last commit, but I consider first and second commits as a must-have, otherwise discovery won't work for the nodes with UnknownCapability (which is not the problem for NeoGo).

Type of change

• Optimization (the change is only an optimization)
• Style (the change is only a code style for better maintenance or standard purpose)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Unit tests

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[Jim8y]

emmmmm, i need sometime to check the p2p part to better understand this pr. But no need to wait for me, just keep going.

[nan01ab]

LGTM