Add a function basemap 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).

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]

A current issue with this is how to handle the projection. I am treating everything as WGS84 right now because base Makie lacks support for attaching CRS. That is actually incorrect since the tiles are returned in web mercator coordinates, which means that stuff like this happens. Until we come to a conclusion on what to do here (reproject to WGS84 or just plot directly in webmerc), this PR will (a) not work right and (b) should not be merged.

[asinghvi17]

You can use this PR with GeoMakie as follows:

provider = TileProviders.Google() # suppose 
fig, ax, plt = image(
    provider, Rect2f((-180, -90), (360, 180)), 1000 #= size =#; 
    interpolate = false,
    source = "+proj=webmerc",
    axis = (; type = GeoAxis)
)
coastplt = lines!(ax, GeoMakie.coastlines(); color = :black, linewidth = .75)
fig

[asinghvi17]

Closing in favour of #91