Add user-defined metadata field to dispatches

[Marenz]

Dispatches currently have a payload field for operational parameters (power settings, SoC limits, etc.), but there's no dedicated field for arbitrary user-defined metadata.

Use cases:

• Storing context from external systems
• Adding tracking/correlation IDs
• Attaching non-operational data that shouldn't mix with payload

Proposal: Add a google.protobuf.Struct metadata field to DispatchData:

message DispatchData {
  // ... existing fields ...
  
  // User-defined metadata.
  // Arbitrary JSON data for client use, separate from operational payload.
  google.protobuf.Struct metadata = 9;
}

Same constraints as payload would apply (50KB max, 5 levels nesting).

This is additive and backward-compatible.

Alternative: map<string, string> labels - simpler and easier to query/filter, but less flexible.

[Marenz]

Related: https://github.com/frequenz-io/frequenz-actor-electricity-trading/pull/580 - adds a BatteryPowerDispatcher that creates dispatches from optimized battery schedules. Currently it has to track dispatches it created by keeping its own list (that part is not coded yet)