Add barebones Remote object, make _caching private

[AbrilRBS]

Changelog: Omit
Docs: conan-io/docs#3962

First idea for Remote model documentation. A few ways to do this in the long run:

• Use Remote typing where appropiate, the docs will link them automatically in the signature
• Use the :ref: styntax from sphinx so docs get linked properly

[CLAassistant]

All committers have signed the CLA.