Add Fallback URLs (for #1203)

[TesteurManiak]

Fix #1203

• Updated NetworkTileProvider and NetworkNoRetryTileProvider from tile_provider_io.dart & tile_provider_web.dart to support a fallbackUrl
  • Replaced references to RetryClient by a simple Client class as I think it is not needed to enforce a retry client, we should simply be able to use any Client class we want (+ allows to use FMNetworkImageProvider inside tile_provider_web.NetworkNoRetryTileProvider.getImage)
• Putted back AssetTileProvider which was missing for some unknown reasons and added support for fallbackUrl by implementing a private class _FlutterMapAssetBundle which will try to load an asset in the same way as the default PlatformAssetBundle, but will also try to load a fallback asset if the first one wasn't found.
• 2 examples have been added: FallbackUrlNetworkPage & FallbackUrlOfflinePage

Changes have been tested on Android and Chrome, no regressions were found.

[JaffaKetchup]

Thanks @TesteurManiak! I'll have a look at this for v3.1.0 :)

[TesteurManiak]

Hi, @JaffaKetchup I've merged the latest changes from master on the branch.
I kept the AssetTileProvider implementation inside it's own separated file with the _FlutterMapAssetBundle. (Seemed better for readability)

[JaffaKetchup]

See the changes I suggested below.
Can you also bring this up to date with v3 if needs be? For example, I know the AssetTileProvider was added back and this might cause problems.

[TesteurManiak]

@JaffaKetchup everything should be ok now 👍

[JaffaKetchup]

LGTM! Thanks.