WF-training: Update STAC item generation

[brunosmets]

See training pipeline architecture diagram

STAC item generation – MANU (L) with support Marcel
• Available in alph3/2
• Add extensions to item & check STAC model properly used
• Collection update based on item and upload
• Integration in NIFI

[manugv]

@brunosmets This is a duplicate of the issues: #160 #179 except the last point

[brunosmets]

blocked by OpenEO core issues.

[manugv]

Only the last point needs to be addressed. Rest has been addressed in the pull request https://github.com/ESA-WEED-project/habitat_mapping/pull/34