Formalize some concepts from STA Plus

[sgrellet]

OGC SensorThings API Extension: STAplus 1.0 (https://docs.ogc.org/is/22-022r1/22-022r1.html) builds on ST API (thus O&M).

There are new important concepts really useful to describe an Observation context (ex : Campaign, ...).

As in the rationale with OMS & ST API V2.0 is to have proper cascade from OMS -> ST API, it would worth considering which concepts from STAplus 1.0 should/could join OMS model (ex : through OMS Logical work on-going).

[sgrellet]

https://github.com/joanma747 : FYI