Tezt/Cloud: Enable DAL "observers" on bakers' topics

What

Add a new CLI argument --dal-baker-observer enabling to run a DAL node on the corresponding bakers' topics.

Why

For debugging peering issues on ghostnet, this can be useful.

How

As usual.

Manually testing the MR

dune exec tezt/tests/cloud/main.exe -- dal cloud --localhost -v --log-file /tmp/log --website --prometheus --grafana --network ghostnet --bootstrap --observer-pkh tz1PZY3tEWmXGasYeehXYqwXuw2Z3iZ6QDnA

And then using

./octez-client --endpoint http://127.0.0.1:30104 rpc get /p2p/gossipsub/topics

you can check you are connected to the topics of this baker as expected.