Allow non primary network validators to request all peers

[cam-schultz]

Why this should be merged

Network.Peers currently only allows primary network validators to request the list of all peers across all L1's. This is primarily a bootstrapping optimization to prevent large numbers of peers for changing validator sets from being circulated as the P-Chain is synced.

Removing this check would simplify peer discovery for non-validators that need p2p connections with arbitrary nodes. One example is ICM signature aggregators - with the current semantics, they are only able to request peers for tracked subnets, which is capped at 16.

How this works

Removes the above restriction.

How this was tested

CI, added unit test

[marun]

I'm assuming this change is desirable, I'll leave it to others to confirm that.