Set aws.vpc.flow.start timestamp field to ISO8601 format

[MichaelKatsoulis]

Description

This PR updates the aws.vpc.flow.start field type and value format for better standards compliance.

Current State:

• Field: aws.vpc.flow.start (integer)
• Value: Unix timestamp in seconds since epoch (e.g., 1609459200)

Problem:
The field represents the start timestamp of a VPC flow log entry. AWS provides this value as seconds since epoch, but OpenTelemetry semantic conventions don't currently define timestamp fields using this format. The majority of timestamp fields in semconv use either ISO-8601 string format or nanoseconds since epoch.

Solution:
This PR converts the field to:

• Field: aws.vpc.flow.start (string)
• Value: ISO-8601 formatted timestamp (e.g., "2021-01-01T00:00:00.000Z")

Benefits:

• Better alignment with semantic convention patterns
• Human-readable timestamp format
• No additional mapping required by downstream consumers
• Consistent with other timestamp fields in the ecosystem

Example:

• Before: aws.vpc.flow.start: 1609459200 (int64)
• After: aws.vpc.flow.start: "2021-01-01T00:00:00.000Z" (string)

Related Issue

• 43392

[axw]

@MichaelKatsoulis given that this has been out in the wild for a little while, I think we should put this change behind a featuregate

[MichaelKatsoulis]

@axw I created a feature gate