doc. encryption: Key managerment service

[islamaliev]

At the moment the doc encryption key exchange is implemented in a very straightforward way: nodes directly communicate with each other over pubsub.

We need to abstract that into a dedicated KMS with different implementations: the current one, one that works with Orbis and whatever else.

On the start a node should provide it's identity, ACP instance and any other required dependencies to KMS so that it can act on behalf of the node.