Add support for Thunderforest tiles

[Guymer]

Rationale

I wanted to use Thunderforest tiles in my own projects and I wanted an easy way to add them to my Cartopy projects. Looking at the Cartopy source code for other third-party tile resources, such as "Stamen", it did not look too daunting to add another one.

Implications

There are no implications to this change - no pre-existing features are affected in any way. This Pull Request simply adds another third-party tile resource.

Here is an example map using landscape tiles at @2x scale (with a higher-than-default value of regrid_shape) using my own personal Thunderforest API key (it shows four day hikes emanating from Finse train station, coloured by walking speed):

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