Add gossip_target configuration option

[OlivierHecart]

Add gossip_target configuration option.

This options allows to restrain the type of zenoh instances (router, peer) the configured instance sends gossip messages to.
This may typically be used in rmw_zenoh to make peers do gossip with the router but not between each other saving a lot of traffic and computation.

[fuzzypixelz]

I tested this with 1 router and 10 peers.

With scouting/gossip/target:["router"] on the peers:

$ tshark -r out.pcap -d tcp.port==60000-70000,zenoh -Y 'tcp.port != 7447' | grep -c 'OAM'
0

Without scouting/gossip/target:["router"] on the peers (i.e. the ["router", "peer"] default):

$ tshark -r out.pcap -d tcp.port==60000-70000,zenoh -Y 'tcp.port != 7447' | grep -c 'OAM'
139

[oteffahi]

Added some comments regarding default config