Create first version of user request payload and surrounding job model

Define first version of request data models:
- request payload model (what is sent from user to api)
- execution_metadata model
- job information model created at runtime (job_id, user_id etc)

These would then be combined to build the complete job record (future stored in sqlite database)

Example job:
```json
{
"job_id": "uuid",
"user_id": "str",
"created_at": "timestamp",
"status": "str",

"request_payload": {
"aggregate_field": "str",
"registry": "str",
"aoi": "RegistryDataset",
"geojson": "GeoJSON"
},

"execution_metadata": {
"s3_path": "str",
"engine_version": "str",
"started_at": "timestamp",
"completed_at": "timestamp"
}
}
```