load_url vector cube: spatial filtering at load time

[jdries]

Support spatial filter pushdown, for formats like flatgeobuf.
The goal is to support reading a part of large files, without going OOM.

[JeroenVerstraelen]

• Can be pushed to Q4, ask Martine first
• Make an up-to-date estimate on how much work this takes

Context: if they load in a large vector data file then openEO will crash