get connected hypervisors

[mrpalide]

Did you run make format && make check? Yes

Fixes #1306

Changes:

• add new command to skywire-cli visor as chvpk that return connected hypervisor PKs

Note:
we have hvpk that return all hypervisor PKs that available in config file, but this command return list of connected hypervisor PKs

How to test this PR:

• make build
• add two hypervisor PK to config, once available to connect and other be mock
• run visor
• check hvpk by skywire-cli visor hvpk
• check chvpk by skywire-cli visor chvpk
• should seems like:

  [mohammed@manjaro skywire]$ ./skywire-cli visor hvpk
  [0218905f5d9079bab0b62985a05bd162623b193e948e17e7b719133f2c60b92093 036c0e89735c675abce702a3d46ad1bf736bd8236dbd734bfaf368959de2c2b967]
  [mohammed@manjaro skywire]$ ./skywire-cli visor chvpk
  [036c0e89735c675abce702a3d46ad1bf736bd8236dbd734bfaf368959de2c2b967]
  

[0pcom]

works as described