Make TileProvider an interface (or: maybe this is just an FYI rather than a PR...)

[thisisaaronland]

I recently had to fork go-staticmaps and change the default TileProvider struct in to an interface (and update things accordingly) in order to it with so-called "rasterzen" tiles:

https://github.com/whosonfirst/go-rasterzen

These are (Nextzen) vector tiles by default that get rendered in SVG/PNG tiles in code so the default model of fetching a URL (in tile_fetcher.go) and decoding the bytes doesn't work for rasterzen tiles. It's also meant moving the actual HTTP request in the tile provider which is maybe messier than you'd like:

https://github.com/whosonfirst/go-whosonfirst-staticmaps/blob/rasterzen/provider/rasterzen.go

Anyway, I just wanted to mention that there was a need to do this. If there's a different/better way to accomplish the same thing then I am happy enough to change my code.

The end result is this:

https://github.com/whosonfirst/go-whosonfirst-staticmaps/tree/rasterzen#wof-staticmap

Also, thanks for writing go-staticmaps in the first place :-)