Provide default options for secret key storage

[rklaehn]

Not sure whether we should do this at all, but... when I look at the workshop code and the code of all my little examples, one thing that I always have to put into the util.rs is some way to make the keypair persistent, either in a data directory or in an env var.

Maybe we should add this to the endpoint builder as well?

[flub]

Isn't this an endless list of new demands? Storage should be encrypted, use the system keyring, use a HSM, ...

I still like the current distinction of not having any storage in iroh-net but allowing things to be passed into the Endpoint::builder().