Defining handler for webhook build worker

Created by: susantoscott

This handler handles records for when a webhookBuildJob is dequeued. This PR contains:

• definition of the handler webhook_build_handler.go
• unit test for the handler webhook_build_handler_test.go
• vcr file for testing

This PR is built on top of the DB schema in this PR which also has a lot of comments resolved already, and the Job in this PR.

Test plan

go test-run TestWebhookBuildHandle