Support for API authentication about inkmap

[jahow]

I guess we could add an option for custom headers for each layer that relies on HTTP requests. For query params, including them in the provided url should be enough!

[jahow]

Just started looking into this but adding HTTP headers to WMS/WMTS layers for example might be trickier than I thought. We're relying on native img elements to load the tiles, and there's no way to specify custom headers with that method. This means we'd have to switch to loading the image data ourselves using fetch, which will be more work.

@jussih could you please confirm that this is needed, e.g. in the case of a tile service protected by Basic Auth? thanks!

[jussih]

For the current sources that I'm using this is not needed.