Add operator entity to groups and add possibility to use Client IPNS keys

[MicrowaveDev]

Currently IPNS keys are storing in GeeSome node, not in the GeeSome Client. But need to use it only as option. Another option - use client keys for sign IPNS and send signed messages to node. Operator - its IPNS key that storing in GeeSome node. Its just a field in IPLD of Group that tells everyone - that IPNS key can post updates about my Group, trust him.

Actually GeeSome node - it is operator, and it can post updates about Groups as operator until GeeSome Client change the operator to another one by posting new IPLD with new operator IPNS.