feat(federated): allow to pickup a specific worker, improve loadbalancing (#3243)

* feat(explorer): allow to specify a worker target

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* feat(explorer): correctly load balance requests

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* feat(explorer): mark load balanced by default

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

* fix: make sure to delete tunnels that might not exist anymore

If a worker goes off and on might change tunnel address, and we want to
load balance only on the active tunnels.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

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Signed-off-by: Ettore Di Giacinto <mudler@localai.io>