Allow users to update fake backends with latest data

[jyu00]

What is the expected feature or enhancement?

When we introduced local testing mode, we encouraged people to use fake backends corresponding to the real target backend they will ultimately use. Fake backends were originally designed to be used completely offline, and their data files (such as backend properties) are never updated once created. This means the noise model of a fake backend can be very different from its real counterpart.

Some users requested that we provide a way to pull the latest backend files, so the noise model would better represent the real systems.

Acceptance criteria

• Add a refresh() method to fake backend that'd pull the latest backend data files from its real counterpart, if one still exists. It'd need to take a service parameter since we need an account to access the real backend.

[jyu00]

I'm re-opening this because the PR doesn't actually "refresh" the existing backend target if it's already constructed (see my comments in the PR).