Create summary of cached content

[AbrilRBS]

Changelog: Feature: Create summary of cached content.
Docs: Omit

Co-Authored-By: @memsharded

Initial work towards the sources backup feature, this PR aims to implement a simple "summary.json" for sources downloaded by a conanfile, which stores info about which reference downloaded which URL, relying in the sha256 checksum as the key.

It also changes the cache layout a bit by moving the mentioned downloaded files by the conanfile to their own s/ subfolder, and the rest to c/

Omitted docs because it only makes sense to document this once the whole feature lands

[memsharded]

Looking good. Not sure what would be the best flow, if we should merge, or we should target "develop2" for 2.1, or how to approach it. Ideally, this part of the cache could start to be used asap I guess

[memsharded]

Note for ourselves: we need modes of behavior for the caching, like being able to raise if the thing is not cached, to raise if it cannot be cached because of no sha256, etc. For future work.

[AbrilRBS]

Changed to always use "unknown" as an origin if the ref is None, and added a test path for when the ref is actually there, but have a question about the usage of the --format=json output