cmd/devp2p: add commands for node key management (#21202)

These commands mirror the key/URL generation functions of cmd/bootnode.

    $ devp2p key generate mynode.key
    $ devp2p key to-enode mynode.key -ip 203.0.113.21 -tcp 30304
    enode://78a7746089baf4b8615f54a5f0b67b22b1...