Configure geo-search fields

[curquiza]

As discussed here

• Geosearch naming: keep or not `_` in suffix? #441 (comment)
• Geosearch usage #439

It could be a nice addition to let the users set the fields Meilisearch should use in the document to process the geo-search.

2 possibilities

• let the users define the whole field, I mean the field containing the object with lng and lat inside. Ex: geo
• let the users define the latitude and longitude fields. Using the nested fields notation it could be geo.lat and geo.long