use new join service for host joins

[nklaassen]

This PR updates the host join process in lib/service/connect.go to call the new lib/join/joinclient.Join instead of the legacy lib/auth/join.Register. This will attempt to use the new join service being implemented as part of RFD 27e where the Auth service noaw assigns the host UUID instead of allowing the joining agent to choose it. Currently only the token and bound_keypair methods are supported by the new join service, for all other join methods or when connecting to older auth servers without the new join service the implementation will fall back to joining via the legacy service.

[marcoandredinis]

master is v19, right?
So, according to the comment we should just delete this

However, what do you think about keeping this for at least one more release?
Or keep it behind a feature flag.

We can discuss it when actually removing it, but this seems to be a pretty important part to just remove it without keeping a way to add it back for at least one major release.

[nklaassen]

theoretically as long as the new join service is fully supported by v19 then we can delete this on the client side in 19.0, but I can't delete it yet because it doesn't support all join methods yet.

But it should be fine to keep it as a fallback on the client side until 20, since v19 servers will still support it, I'll update the DELETE IN comment

[backport-bot-workflows]

@nklaassen See the table below for backport results.

Branch	Result
branch/v18	Create PR