Clarify how state hashes are used between the Agent and the Server

[tigrannajaryan]

Resolves https://github.com/open-telemetry/opamp-spec/issues/76

• Added a section that explains the state synchronization from the Agent to the Server.
• Added flags for Server to ask for AgentDescription and RemoteConfigStatus and dded hash field to AgentDescription and RemoteConfigStatus. This makes all 4 state messages consistently use the same state synchronization approach.