ENH: support xyzservices

[martinfleis]

Closes #1497

The PR is ready, we just need to wait for xyzservices 2021.08.0 to be released as it currently requires functionality available only on main. That shouldn't take more than a couple of days.

[martinfleis]

Looking at your management of built-in tiles, there's a possibility to simplify all that by relying on xyzservices directly.

In TileLayer we can map those 10 currently built-in on relevant TileProvider objects and get rid of all that complicated machinery you have to manage templates/tiles. We could even map any custom non-URL string to matching TileProvider (as "esriworldtopomap" would be mapped to xyz.Esri.WorldTopoMap, essentially expanding the number of built-in tiles to the whole library we have in xyzservices. That would essentially outsource the maintenance of tile sources to xyzservices.

It would require having xyzservices as a hard dependency though but keep in mind that it itself has no deps.

It is up to you. I can either expand this PR or make the refactor in another one if you'd like to have it.

[martinfleis]

xyzservices 2021.08.0 is now available on PyPI so the tests should now pass.

[Conengmo]

Nice integration. Let's merge it when the tests pass.

Additionally, we might want to include an example in our Quickstart example notebook or perhaps a separate notebook.

[martinfleis]

Additionally, we might want to include an example in our Quickstart example notebook or perhaps a separate notebook.

Do you want it to be a part of this PR or a follow-up?

[Conengmo]

Let's do it as a follow-up, since this one is good to go!