Add a function basemap to get a static image from Tyler, and some converts for image

[asinghvi17]

This file provides the ability to get static base maps from Tyler.

Its main entry point is the basemap function, which returns a tuple
(x, y, z) of the image data (z) and its axes (x and y). The image
is in a web-mercator CRS, we need to figure out a way to also get eg WGS84 tiles.

This file also contains definitions for convert_arguments that make
the following syntax "just work":

image(TileProviders.Google(), Rect2f(-0.0921, 51.5, 0.04, 0.025), (1000, 1000); axis= (; aspect = DataAspect()))

You do still have to provide the extent and image size, but this is substantially better than nothing.

[asinghvi17]

(This doesn't work now but will work with #90, since that imports FileIO)

[rafaqz]

Is there a reason basemap isn't in MapTiles.jl ?

Would let other packages skip the Makie dep

[asinghvi17]

HTTP.jl and ImageIO.jl are required to stitch the tiles together, but if those can go in MapTiles I'm fine to upstream this

[rafaqz]

Ahh right. Might be worth it for Plots.jl uses

[asinghvi17]

This will have to be reimplemented based off #95