Additional support for LINZ Maps Tiles

[aluthfian]

This is to add support for plotting NZ Map tiles, which is in EPSG:3857 (https://epsg.io/3857) already and in png format. Layer ID can be read in every layer's "About" tab, and the "subscription key" parameter is actually an API key given to the registered users. Tested in Python 3.8 and runs well.

Rationale

To add support to plotting LINZ Map Tiles, which are used widely within the Australia-New Zealand region.

Implications

More cartopy applicability for Australia-New Zealand users.

[greglucas]

Is there a way to add a test for this if we have an environment variable for the subscription_key set? See here for a similar situation:

cartopy/lib/cartopy/tests/test_img_tiles.py

Lines 287 to 295 in ba25aee

	@pytest.mark.network
	def test_azuremaps_get_image():
	# In order to test fetching map images from Azure Maps
	# an API key needs to be provided
	try:
	api_key = os.environ['AZURE_MAPS_SUBSCRIPTION_KEY']
	except KeyError:
	pytest.skip('AZURE_MAPS_SUBSCRIPTION_KEY environment variable '
	'is unset.')

[greglucas]

@aluthfian it looks like you haven't signed the CLA: Blocked: CLA needed See https://scitools.org.uk. Submit the form at: https://scitools.org.uk/cla/v4/form

Looks good though. My only other minor comment is that right now we have "API_key", "apikey", "subscription_key" as variable names for the services. I'm wondering if we should follow precedence here and use "apikey" for this as well? Or change the others at some point for consistency.

[aluthfian]

Yup, done! Thanks for reviewing!

Fian

[greglucas]

Thanks, @aluthfian! Sorry for not checking on this in quite a while!