Optionally show or hide Bing 'placeholder' tiles

[Shane98c]

Fixes #14541

Placeholder tiles are shown by default for different styles as described in the issue. The behavior is controlled by the presence of the n=z url param. This PR adds an optional parameter allowing users to decide whether to show these tiles or, alternatively, overzoom the best available resolution. If the parameter is not set, the default behavior for the style is used.

This is a fairly big improvement from the method of using maxzoom to hide placeholder tiles mentioned in the example because Bing's zoom level coverage differs across the globe, so while a maxzoom of x works to hide the tiles in the US, a different one is required in, eg, Spain.

[github-actions]

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[jahow]

Thank you for the contribution! That feature is definitely useful. I've made a couple of remarks regarding the implementation, because I think it can be simplified.

Do you think you'd be able to add a test covering this feature here?

openlayers/test/browser/spec/ol/source/BingMaps.test.js

Line 25 in b80fc19

describe('#tileUrlFunction()', function () {

Thanks again

[Shane98c]

Thanks for the thoughtful review @jahow! All makes sense.

I've made the suggested changes and added a few basic tests. I'm new to using karma, so any pointers on that front would be very welcome.

[jahow]

I think it's better now, thanks!