Configurable and extendable IR node validation checks

[roman-khimov]

Is your feature request related to a problem? Please describe.

Current node validation logic mixes logical correctness and policy checks. For example, address validation is just a correctness checks, same thing for maintenance mode. However locode check is a policy, networks may want to do it or may not want to. They at the same time may want to check some network-specific attributes according to any network-specific logic.

Describe the solution you'd like

Make embedded policy checks configurable and provide a generic extension hook to check node data via an external program.

Describe alternatives you've considered

Embedded ones can be moved out of the node as well, but maybe that'd make them a bit less convenient to use.