Send the client version in the header

[nhoening]

... , so the server can in principle decide on feature switches.

Maybe FM-Client-Version would be a good name.

[Flix6x]

Should we make a sister issue on FlexMeasures to store the version as a user attribute?

[nhoening]

Not sure why we'd need that information longer than the current request?

[Flix6x]

To facilitate checking which versions are in use across the clientele.