Make webhook timestamp timezone-aware

[psteffes]

Environment

• Python version: 3.7.5
• NetBox version: 2.6.12

Proposed Functionality

My proposal is to convert the webhook "timestamp" attribute to a UTC-aware ISO 8601 string - 2020-01-24T14:22:26.976422+00:00. This can be done with the already included datetime.timezone, django.utils.timezone, or pytz.

Use Case

Timestamp can be used to ensure the correct processing order of webhooks, and the timezone-naive timestamp is ambiguous. Switching to a UTC-aware timestamp follows standard best practice around datetimes and removes any ambiguity for webhook consumers.

Database Changes

None.

External Dependencies

None.