Add service package

[moorereason]

To help facilitate new features, begin moving the main webhook service properties to a Service struct.

[moorereason]

@adnanh, This PR is a preview of something I've been slowly whittling on. It all started by wanting to fix #393. I needed to be able to access the http.Server from the signal handlers.

This PR doesn't fix #393, but it does lay the groundwork to make that rather easy (see the KPR in the security package). However, there's a lot of changes here which may mean that I need to split this up into smaller steps so that it's easier to review, but I wanted to let you see where I'm headed.

It looks like adding the TLS Curves is a problem in Go 1.14 since the CurveID type didn't support the Stringer interface until Go 1.15. Instead of trying to work around that will build tags, I'll probably wait until Go 1.16 ships, and we can drop Go 1.14. 😃

No rush, but comments are welcome.