Change log for GCP_VPC_FLOW

Date Changes
2023-05-23 Enhancement -
- Mapped 'metadata.event_type' to 'USER_RESOURCE_ACCESS' when field 'logName' does not contain 'vpc_flows'.
- Mapped 'timestamp' to 'events.timestamp'.
- Mapped 'textPayload', 'labels.tunnel_id' to 'additional.fields'.
- Mapped 'resource.labels.region' to 'target.location.country_or_region'.
- Added null checks for various fields wherever required.
2023-04-10 Enhancement -
- Set "target.resource.attribute.cloud.environment" to "GOOGLE_CLOUD_PLATFORM".
- Set "target.resource.name" to the full resource name value.
2022-07-22 Enhancement:
- Mapped "resource.labels.location" to "principal.location.name".
- Mapped "resource.labels.subnetwork_id" to "target.user.attribute.labels".
- Mapped "logName" to "security_result.category_details".