Add support for GVA subscription

In "Prototype de GVA" forum thread, the subscriptions are now served on GVA endpoint type with an additional /subscription path to the specified path /$gva_path in the GVA endpoint.

Introduce helper(s)
Introduce example
Add support for /$gva_path/subscription path. $gva_path is found in GVAEndpoint`

Edited Jun 21, 2021 by Moul

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