Add cache headers to (configuration) RPCs

[kschiffer]

Summary

Our configuration endpoints should contain cache information, so that the console can cache these values and avoid redundantly re-requesting. This is currently e.g. the case for frequency plans, which will be refetched on every page load (of views that requre them).

Why do we need this?

Avoiding unnecessary api calls and increasing the perfomance of the console.

What is already there? What do you see now?

The console already has caching capabilities, which are however without any invalidation logic and are since not currently used.

What is missing? What do you want to see?

Response headers that allow for proper cache invalidation rules, e.g. Expires/If-Modified-Since/ETag headers.

Environment

Various.

How do you propose to implement this?

Adding above headers to the request responses of our (configuration) RPCs.

Can you do this yourself and submit a Pull Request?

I think @htdvisser is better suited for this, I can do the console part. Anyone, feel free to pick up or reassign.