Defining the handler for background webhook build worker

Created by: susantoscott

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

• definition of the handler
• unit tests for the handler
• testdata and vcr files for testing
• store.go and mocks_temp.go changes to obtain user data for authentication
• go.mod and go.sum for secret generator using the randstr library

This PR is built on top of the DB schema in this PR and the Job in this PR.

Test plan

go test-run TestWebhookBuildHandle