[REQUEST] Add Belgium Wallonia

[thiagovmdon]

Data source
SPW Hydrometrie (Wallonia, Belgium)
Main portal: https://hydrometrie.wallonie.be/
KiWIS API base URL: https://hydrometrie.wallonie.be/services/KiWIS/KiWIS
KiWIS request reference: https://hydrometrie.wallonie.be/services/KiWIS/KiWIS?service=kisters&type=queryServices&request=getRequestInfo&format=html

Type of data portal
Data are downloadable programmatically via HTTP GET requests returning JSON.
No registration is required for the endpoints used here.
The portal provides both station metadata access and time series value access.

Country/Countries/Regions
Belgium, Wallonia region.

Variables
The portal clearly exposes at least these variables used in RivRetrieve:
Daily discharge
Daily water level / stage

Station list
https://hydrometrie.wallonie.be/services/KiWIS/KiWIS?service=kisters&type=queryServices&request=getStationList&datasource=0&format=json&returnfields=station_no,station_name,station_latitude,station_longitude,site_name,ca_sta&ca_sta_returnfields=

Code snippets
Code already implemented in branch Belgium-Wallonia