Reference placeholders

[emmanuelmathot]

This PR adds the ability to reference item, collection and asset reference in the URLs

[m-mohr]

Related to and should probably wait for #10

[m-mohr]

Also, I'm not really getting why these values are not hard-coded? In the example it looks like all values are "fixed" to a specific value (except for asset)?! And then, why is the asset key defined but not the asset href and/or type?

[emmanuelmathot]

Also, I'm not really getting why these values are not hard-coded? In the example it looks like all values are "fixed" to a specific value (except for asset)?!

right, the item values could be fixed as the link is part of it. But there is still the asset case

And then, why is the asset key defined but not the asset href and/or type?

because the asset is relative to the item and it's href may be computed form several source (e.g. alternate extension). a tiling service such as titiler read the stac item and use the asset name to make the tile. The asset placeholder offers the possibility to list a unique link for the tiling service for the item (including all assets)