xcube extensions should have a version identifier

[forman]

Is your feature request related to a problem? Please describe.

We often need to know the version of a certain xcube data store extension. This is not easy, because

1. version info is associated with the contributing plugin only
2. we don't know the contributing plugin

Describe the solution you'd like

The version of an extension does not necessarily reflect the version of the plugin that contributes it. However, we should add extension metadata "version" field to each extension and let it fallback to the plugin version. Passing the "version" value or plugin version would be responsibility of the extension developer. The "version" field is already in use by xcube server API extensions.