Fusion: Rework Stamen tile layer support

[jumpinjackie]

Stamen tiles is now under the management of Stadia Maps, which requires API keys (or domain whitelisting) to access their services.

As a result the way we currently set up Stamen tiles in our Fusion flexible layout is no longer valid and needs to be updated to support the new way of accessing these tiles, which would be a XYZ layer with placeholders for x, y, z and api key query string parameter.

Or alternatively, we stick with the current Stamen layer type, but add runtime support in Fusion to set up XYZ layers with the right URLs, API keys and attributions.

The API key field should be denoted such that it is not required if their Fusion application is hosted on a domain whitelisted by Stadia Maps (link to relevant documentation on this)

https://docs.stadiamaps.com/guides/migrating-from-stamen-map-tiles/