made tileLayer accept async buildTileUrlTemplate function

[randyberos]

This enables async function for the buildTileUrlTemplate option.

Will solve the issue related to this.

[DanielJDufour]

Awesome. On the face of it, looks great! Could you add a test just to double check? Thank you!

[m-mohr]

Just for awareness: This PR is against the codebase for the stac-layer rewrite which will likely will be released as 1.x, but it may never appear in STAC Browser, which is still on the old codebase (0.x). It depends a bit on funding whether STAC Browsers next version will be using stac-layer 0.x, stac-layer 1,x, or ol-stac 1.x.

[randyberos]

Changed the tilers/titiler/build-tile-url-template.html test to use async buildTileUrlTemplate. Test on rendering the COG using buildTileUrlTemplate still works as intended.

[m-mohr]

I updated the README to reflect the change, too.