New edition of the webapp protocol

[glpatcern]

Following discussions in the mailing list, this is the current proposal, for now only in terms of OpenAPI - the I-D is to come after we have consensus on this one.

A short summary:

• The Discovery endpoint now includes -receive protocols, to signal the ability for an endpoint to receive a share (whereas so far what was advertised is the ability to send and serve shares). This was implicit with webdav, but it's not for other optional protocols.
• Specifically for webapp, the webapp-receive protocol includes a targets, to signal all possible targets the endpoint can support.
• The webdav-uri capability has been moved into a uri attribute of the webdav-receive protocol, following the new schema. This was never used so far, so such breaking change is not expected to actually break any implementation.
• The webapp share now includes targets, permissions (replacing viewMode), appName, and appIcon.
• Absolute URIs are not deprecated any longer.

Questions:

• Is there any better idea to represent the -receive protocols, whilst preserving backwards compatibility for the webdav protocol?
• Does the webapp share payload capture all that was discussed so far?