Allow collections to be flagged as experimental

[soxofaan]

Most resources (processes, file formats, service types, runtimes, ...) can be flagged as experimental, but that's not possible with collections apparently.

Not sure if this is intended for some reason (STAC compatibility?) or an accidental oversight

This PR tries to fix that

[soxofaan]

Ok, I found some pointers to discussions on this topic:

• Publish maturity level of processes + collections #445
• https://github.com/openEOPlatform/architecture-docs/issues/323
• initial collection requirements openEOPlatform/documentation#70
• https://github.com/openEOPlatform/architecture-docs/issues/325

But most of these tickets are non-public, so hard to discover.
Most of the discussion was also in the context of openEO Platform, while the value of this flag has a larger scope than that.

Regardless, one of the public outcomes (currently at https://docs.openeo.cloud/federation/backends/collections.html#collection-availability):

The 'experimental' flag from https://stac-extensions.github.io/version/v1.1.0/schema.json needs to be set on experimental collections. A collection can be experimental either because there are issues with the actual data or catalog, or because of backend specific issues that make the use of the collection unstable. A collection marked as experimental does not need to comply with further requirements

So in conclusion, the "experimental" flag can already be added through the "version" STAC extension, so the need to explicitly add it in the openEO API spec too is pretty low.
Counter-point: the "deprecated" flag is also part of the "version" STAC extension, while it is also explicitly listed in the openEO API spec, so there is a bit of asymmetry here.

[m-mohr]

We can add it to the spec, I think. We use it in other areas as well. The Web Editor even makes use of it already.