Add anglebands to SENTINEL2_L2A.

[EmileSonneveld]

Open-EO/openeo-geotrellis-extensions#177

[jdries]

we'll want to use different band names to comply with:
https://docs.openeo.cloud/federation/backends/collections.html#sentinel2-l2a
you can do something like:
{
"name": "viewZenithAngles",
"aliases": ["S2_Level-2A_Tile1_Metadata##3","VZA", "viewZenithApproximate"],
}

It could be that in this case we only have the approximate angles.