Wip/add landcover example

[abarciauskas-bgse]

@emmanuelmathot @vincentsarago @smohiudd it was proposed in the STAC sprint (see stac-extensions/web-map-links#19) that this composite meta-extension could solve the needs of the proposed render extension. I think this makes sense but there are a few concerns I have:

1. communication - how do users of STAC and titiler know how to use this extension properly? It's still not entirely clear to me (as you will see in this PR) how to use this extension to satisfy the use case of rendering raster datasets via the titiler API.
2. currently the composite extension is on the item level - I'm not sure we need to hydrate all items with these values so could it also be at the collection level?
3. Right now the processing extension only allows for objects to be the value of the processing:expression and, as you see in this PR, what I think we need is a list.

[vincentsarago]

oh I forgot about this extension!

It seems a bit more complex than the render one but 👍

communication - how do users of STAC and titiler know how to use this extension properly? It's still not entirely clear to me (as you will see in this PR) how to use this extension to satisfy the use case of rendering raster datasets via the titiler API.

IMO, it's not up to titiler to know how to use this but to the client/user to construct URL correctly reusing the information.

currently the composite extension is on the item level - I'm not sure we need to hydrate all items with these values so could it also be at the collection level?

It would make sense to have this at collection level IMO

[abarciauskas-bgse]

@emmanuelmathot options for expanding (or not) the processing spec to fit this use case:

1. make processing:expression also accept a list, like I've done in this example
2. create a new field for processing, perhaps processing:functions which accepts a list of key/value pairs with "name" for name of the function (i.e. rescale) and value (i.e. -1,1)
3. use processing expression as-is where format is something generic but identifiable for dynamic tiling like render-params and then the expression is a query string rescale=-1,1&colormap=rdbu_r etc

I don't really like 3 since it doesn't make it easy to programmatically work with those values.

[abarciauskas-bgse]

Closing in favor of stac-extensions/processing#28