feat: coordinated mix-net transmission round config file

[xendarboh]

Current State

• mix-net transmission rounds are configured by many --args
• command args works ok for testing, but unwieldy

Desired State

• within context of protobuf and (un)marshalling to/from JSON
• manage configuration within a single config file
• consider context within complete configuration for a transmission round including servers, groups, gateways, etc
• consider other meta data including timestamp, UUID, etc
• initially coordinator publishes, plan for decentralized publication post-coordicide