Check node connectivity during joining

### Is your feature request related to a problem?

When a node joins the network, it submits a transaction indicating its connection endpoints (IP, ports).
Currently, we are not checking the information on its connection is right, or the node is reachable.
So, the node could join the network but not reachable since the beginning. (This could be seen as spam because other nodes would have to request liveness of this node in the BeaconChain)

### Describe the solution you'd like

To prevent this annoying situation, I propose to make a check in PendingTransactionValidation to ensure the node is reachable to the IP/ports given, and is not providing false information.

### Additional context

Epic: #306

Start by locating PendingTransactionValidation and read the context of epic #306. Determine how the joining transaction supplies its IP and ports and how reachability can be checked before acceptance. Done means a node advertising unreachable connection endpoints cannot successfully join the network.

Written by the indexing model from the issue text.