auto-peering visors to the hypervisor (skybian)

[0pcom]

attempt to make visors re-peer to a new hypervisor when a new hypervisor is present at a predetermined static ip

Fixes #1312

this PR ports the existing auto-peering functionality from skybian to the skywire source code / binaries and attempts to extend this system to re-peer visors to the hypervisor when a connection is lost

testing requires a hypervisor running the following:

skywire-cli visor pk -w

The srvpk endpoint may be used instead of the above command

run the visor from source as follows:

go run cmd/skywire-cli/skywire-cli.go config gen -brn | sudo go run cmd/skywire-visor/skywire-visor.go -l "go run cmd/skywire-cli/skywire-cli.go hv pk -i 192.168.0.100" -mn

replace the ip address in the above command example with that of the running machine.

[0pcom]

to test without using the default http endpoint for the public key (or any)
one can create a file that contains the public key of a running hypervisor for which re-peering will be attempted

echo -n "024034dd09cbff03787db740124496ce8fdbe0cac2a51e20366cb59be10b665506" | tee skywiretest.txt

then invoke the visor as follows:

 sudo go run cmd/skywire-visor/skywire-visor.go -pml "cat '/home/user/skywiretest.txt'"

as you run this visor in the foreground, turn the (remote) hypervisor off and on to observe that the command string provided as an argument to the -l flag is executed on redial attempts to the hypervisor.

To further test, one can edit the file to contain a different public key while this is happening, save the file, and it should then attempt to redial with the new public key.

this needs to be made versatile enough to handle a list of remote hypervisor public keys. Ideally, the http endpoint would output the keys in the same format that they are read, to require minimal parsing.

[jdknives]

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