Light-weight asynchronous invocation model

[pvretano]

Not sure if this should be included in the document somewhere but work has been done, since Catalogue 3.0 actually, on a light-weight asynchronous invocation model for OGC API web services. The latest work can be found here:

https://docs.opengeospatial.org/per/18-045.html#async_extension

If this work is relevant for this document, I would be happy to create a pull request with a summary of the work.