Support GML for results

[fmigneault]

Description

Any time a GeoJSON (or STAC) is requested, a GML equivalent can be considered for XML response.

Consider using https://github.com/geopython/pygml to translate and return the XML response.

References

Conversion can allow more coverage of results/output content-negotiation utilities:

• Job output transform #548
• Support /results/{id} and /outputs/{id} routes #18