advertise client-id using the userAgent field from libp2p

[bernardoaraujor]

In order to be able to estimate the diversity of Waku clients, it's suggested that each implementation advertises its client-id using the userAgent field from libp2p (i.e. withAgentVersion(agentString) for nim-libp2p), where in our case client-id is waku-rs.

requirements:

• we want to be privacy-preserving and we don't want this information to be used against the node, just the client type is advertised, without its release, version, OS or any other sensitive information.
• user of waku must be free to update this string easily if they doesn't want to display this information.
• userAgent can be configured with a cli flag, in case someone doesn't want to reveal it.

ref: waku-org/pm#3