Optimize WFS getFeatures depending on the outputFormat

[eblondel]

To optimize the WFS getFeatures, two internal properties are added:

• read_features: required for objects that are not already sf objects, most of the cases, with the exception of CSV output formats for which geometry is empty. In this case, we return immediatly the data as tibble
• validate_features: true for GML WFS default output formats. For JSON and CSV however, we do not need to coerce again data, so it will be set to TRUE.