Add mutual_cluster example

[alexchamberlain]

Add a more detailed example of running multiple processes that mutually authenticate with one another. This demonstrates:

• How to use rcgen and associated creates to generate a toy CA
• How to load certificates and configure connections for mutual authentication
• How to keep track of connections and reestablish them if they go down

There might be a couple of points of clean up on this one, but I think it's good enough to share. I understand this is a slighly bigger example than some of the others, but it took me a while to figure out how to get the crates to play together and I figured others might find it useful; is this of interest?

[djc]

This seems pretty specific for the Quinn repo, and it spends quite a bit of code on things that would be better documented over in the rcgen repo instead of this repo.