[Feature] Improve references to employed schemas in OpenAPI

[fmigneault]

Each schema defined in our OpenAPI spec should refer to the corresponding schema from the OGC-API WPS-REST bindings :
https://github.com/opengeospatial/wps-rest-binding/tree/master/core/openapi/schemas

We could add this information using JSON-LD style fields, eg:

"@context": "https://raw.githubusercontent.com/opengeospatial/wps-rest-binding/master/core/openapi/schemas",
"@schema": "complexData.yml"

Field @context will probably need to be updated eventually to http://www.opengis.net/spec/ogcapi-processes/1.0/conf/core for the official reference, but since it is not yet available, the above raw-github-content will be more descriptive. (see : opengeospatial/ogcapi-processes#71)

We should also display the objects/classes schemas that are defined under OpenAPI UI so they can be viewed individually/independently of the full context within a request/response body. This would ease visibility. At the moment only routes are displayed, but the schemas can also be shown (probably a config parameter to set in the SwaggerUI generator).

We should also apply title field of the employed schemas to match exactly the name of the reference schema from OGC WPS-REST bindings wherever applicable. This would avoid ambiguities.

[fmigneault]

NB: schemas moved under
https://github.com/opengeospatial/ogcapi-processes/tree/master/core/openapi/schemas