Add a distinct option to the db search method

[chaen]

The use case is to be able to return the distinct list of values matching given parameter. This fits nicely in the search method of the DB classes, that needs adding a distinct=False parameter.

• [ ] Add the parameter to the route (see https://github.com/DIRACGrid/diracx/blob/39a73bbd84e8baf96a58d8b7058c2bb0fc1966be/src/diracx/routers/job_manager/__init__.py#L52 )
• [ ] Add the behavior
• [ ] Add the tests