Create background worker to handle building webhooks

Created by: susantoscott

In this PR:

• Defining the background worker which implements the Job interface
• Defining the resetter, cleaner
• Defining the Job struct
• Defining enqueue and dequeue rules
• Handler will be defined in the next PR

This PR uses the schema built in https://github.com/sourcegraph/sourcegraph/pull/39292.

Test plan

Testing enqueue and dequeue functionalities, which involves the worker, DB schema, Job struct No test for Handler go test -run TestJobQueue