Support for API authentication

[jussih]

Some tile sources, like the Maanmittauslaitos WMTS api (page in Finnish only) in Finland, require API authentication to fetch tile data.

In my use case I was able pass the api key as part of the tile url as an url parameter. However it would be more robust to be able to add it as a header for the http requests.

I propose two mechanisms to support api authentication. These could be optional configuration in the printSpec.

• ability to add query params as key/value pairs
• custom headers that will be used for the tile requests

[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.