[Geocodable] Add a function to compute distances in meters.

The current `DISTANCE` function computes distances in miles using the
`<@>` postgresql operator.

This commit adds a function `DISTANCE_IN_METERS` that uses the
`earth_distance` and `ll_to_earth` functions to compute distances in
meters.

It also adds a new method `GeocodableRepository::findByDistanceInMeters()`.