Provide startup/teardown hooks

[ejizba]

Related to #522, which is focused on invocation hooks (needed for distributed tracing), there is benefit to startup & teardown hooks as well

A few differences between putting code at the top level of the startup file vs. a startup hook:

1. Startup hooks can use await (commonjs files don't support top level await, although es6 does)
2. Startup hooks can be passed a context object with extra information about the worker

[hossam-nasr]

Startup hooks added in #577. Teardown hooks tracked by #606. Closing this issue