Implement transaction handling

app/schemas/meter_value.py

@@ -1,5 +1,7 @@
 from datetime import datetime
+from decimal import Decimal
 import enum
+from typing import Optional
 from uuid import UUID
 from pydantic import BaseModel
 
@@ -46,9 +48,9 @@ class MeterValue(BaseModel):
     id: UUID
     timestamp: datetime
     measurand: Measurand
-    phase_type: PhaseType
-    unit: str
-    value: float
+    phase_type: Optional[PhaseType] = None
+    unit: Optional[str] = None
+    value: Decimal
     transaction_id: str
 
     class Config: