Periodically write a hostfile on the innernet server

[bschwind]

Getting this started, spurred by a comment.

Related to #135 and #335

Right now this is implemented as a periodic tokio async task on the server when you run the serve command. I extracted the update_hosts_file() function from the client into the shared crate. That felt like the highest value location to add this functionality, though we could also add it to the various other server commands if we wish.

Beware, I haven't tested this yet at all.

[bschwind]

Ideally we'd just do a SELECT ip, name but the deserialization machinery is all in place to select the whole peer so I just went with that for now for convenience. Performance likely wouldn't be a problem unless you have a lot of peers.

[alerque]

If there is some expectation that this is pulled together enough to maybe work I'll give it a test drive.

[bschwind]

@alerque I would recommend testing it by creating a new network with this server binary, adding a peer, then running the serve command to see if it writes to the hostfile. If you don't get to it I'll test it out later.

[alerque]

I have deployed e57ab94 on a production network by swapping out the system package for the last stable version (I am the Arch Linux packager for innernet) with an otherwise nearly identical package one built from this PR. I swapped out the package, restarted the server, and the /etc/hosts file and directs access to the innernet piers is working as expected! The catch of course is that things like innernet list <network> don't show anything at all and clients show it as innernet-server.<network.wg instead of what might be a preferred pier name. But having the current pier IP list in the hosts file is a huge improvement even without being a fully normalized member itself.

[bschwind]

@alerque nice, thanks for testing! I'll see what I can do about the naming, but glad to hear it at least somewhat works.

[strohel]

Sorry for a late review - this looks good!

Initially I had a slight concern about 10 secs interval being a bit too low, but I see that we have an optimization that we don't write at all if the contents would be the same, so this boild to reading /etc/hosts every 10 secs, which is very fine.

Ship it, especially if this has already received testing.

[strohel]

Rebased on main to get a fresh CI run.

[mbernat]

Tested that serve now writes into /etc/hosts. Let me know if you'd like to test something more specific; otherwise looks good to merge.

marek@Mareks-Laptop innernet % tail -n1 /etc/hosts

marek@Mareks-Laptop innernet % cargo build && sudo ./target/debug/innernet-server serve foo & 
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.07s
[1] 16836
marek@Mareks-Laptop innernet % tail -n4 /etc/hosts
# DO NOT EDIT innernet foo BEGIN
10.47.0.1 innernet-server.foo.wg
10.47.64.1 marek.foo.wg
# DO NOT EDIT innernet foo END

[strohel]

@mbernat thanks for testing, that was is, merging.