Add sync_from option

[caike]

1. Allows syncing from a particular point in the chain.

def start_link(opts) do
  # initial_state = [sync_from: :babbage]
  # initial_state = [sync_from: :origin]
  initial_state = [
    sync_from: %{
      point: %{
        slot: 114_127_654,
        id: "b0ff1e2bfc326a7f7378694b1f2693233058032bfb2798be2992a0db8b143099"
      }
    }
  ]

  opts = Keyword.merge(opts, initial_state)
  Xogmios.start_chain_sync_link(__MODULE__, opts)
end

2. Fixes a bug where the client would suddenly disconnect. By returning {:reconnect, initial_state} from init/1 it appears to fix the bug.