[encoding/googlecloudlogentry] Add fields for MIG + Service metadata

[andrewvc]

Description

WIP/DRAFT! Not yet ready for review

This PR extends the VPC flow parser to emit managed instance group regions and Google service metadata (type, service name, connectivity, optional private domain) for both source and destination flows.

Link to tracking issue

Issue to be created

Testing

• Adds unit coverage for the new fields, introduce new test fixtures for data from these flows
• Adds scripts that can configure a GCP environment that produces these logs to validate that the output here works in the real world

Documentation

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