better document server mode interfaces

[sbrunato]

Update docstrings in rest/server.py to have a more detailed documentation in http://127.0.0.1:5000/api.html, especially for the STAC query extension in POST /search.

For the query extension:

• clear format data validation (ex: list or string)
• supported operators

Use https://github.com/radiantearth/stac-api-spec/ yaml files to update eodag/resources/stac_api.yml:

• https://github.com/radiantearth/stac-api-spec/blob/main/item-search/openapi.yaml
• https://github.com/radiantearth/stac-api-spec/blob/main/core/commons.yaml
• https://github.com/stac-api-extensions/query/blob/main/openapi.yaml
• ...