Add an operation to cache generated files

Is your feature request related to a problem? Please describe

I have a pyinfra deploy which sets up a TLS certificate using acmetool: https://github.com/deltachat/pyinfra-acmetool
It generally works, but if I recreate a VM multiple times, I get ratelimited by Let's Encrypt: https://letsencrypt.org/docs/duplicate-certificate-limit/

Describe the solution you'd like

A solution could be to have an operation combining files.get() and files.put().
We could name this operation files.cache(src, dest).
If the src exists locally, this operation should copy the file to remote host, operating like files.put(). Otherwise it should not do anything immediately, but defer a files.get() operation to the end of the deploy.

For example, destination can be a private key file. If the file already exists, it gets deployed on the server. Otherwise the service is started, detects that the key does not exist and generates it, and pyinfra saves it locally for the next deploy.

Start by reading the existing files.get() and files.put() operations and how deploy operations are deferred or finalized. The operation is done when an existing local source is copied remotely, while a missing source schedules retrieval at the end of the deploy as described.