Add comment on all queries under version control that they are actually under version control

[harisang]

When looking at a Dune query, it is not clear if it exists in this repo. We should add a comment at the first line of each query that is under version control that says something like

-- This query is under version control in this url: https://github.com/cowprotocol/dune-queries/blob/main/cowprotocol/monitoring/solver_approvals_4173928.sql
-- Any change to this query should be done via a corresponding PR in the dune-queries repo.