Metadata files upload/download

[memsharded]

Changelog: Feature: New metadata files associated to recipes and packages that can be uploaded, downloaded and added after the package exists.
Docs: Omit

This proposal implements upload + download flows, but not conan install or conan create installation of metadata. That would add too much extra complexity to the code and uncertainty and fragile developer flows, like metadata not being pulled from server because the package is already in the cache, despite users trying to conan install --metadata=*, thinking that it will pull new metadata from servers.

So probably the best flow to support full-graph download of metadata would be:

• conan install ... --format=json > graph.json
• convert graph.json => pkg_list.json
• conan download --list=pkg_list.json --metadata=logs*

This flow has been already implemented in #13928

Close #13715
Close #4610

[AbrilRBS]

Seems like we'll need to implement the package list inputs sooner rather than later then :)