Identify attached entity lists in the manifest response

[lognaturel]

As the implementer of a data collection client
I want to know which media files are entity lists
So that I can treat them differently from static media files

Form attachments are communicated to clients through a manifest: https://docs.getodk.org/openrosa-form-list/#the-manifest-document

We will add a type attribute to the mediaFile nodes. For now, the only recognized value will be entityList.

• Verify old Collect clients are ok with additional attributes
• Post proposal on forum
• Add to OpenRosa documentation

CC @seadowg

[lognaturel]

We can track the docs item at getodk/docs#1852