Conan private metadata files

[memsharded]

Changelog: Omit
Docs: https://github.com/conan-io/docs/pull/XXXX

Try to move forward, step by step #10261

First we need a Conan-private specific subfolder, that is always retrieved and uploaded, that can contain arbitrary files, not tied to the recipe-revision. Might need some documentation to explicitly document that .conan subfolder is for Conan use only, not modifiable or usable by users?

The intention is that such a folder is used exclusively by Conan implementation, not by users. We might want to discuss if we want a different name, mechanism, location in the cache, etc, to differentiate it?

This could be extended for package binaries too, but at the moment there is no need, the lockfiles-in-recipe only needs recipe metadata.