[pagination] Implement offset

[jeanpommier]

The limit parameter allows to retrieve just a chunk of the dataset. But for pagination to be really usable, we also need

• to be able to set the offset
• to be certain that the features' order will be the same between 2 queries with a different limit+offset setting

[groldan]

offset is used, but the spec doesn't say how to implement pagination, it only tells the response shall contain a link to the next page.
So those links do use an offset query parameter, if you look at a geojson feature collection response, for example

[jeanpommier]

Would it be complicated to add the offset as an explicit param in the OpenAPI form ?