reorganize `cmd/nebraska/`

[vbatts]

there is way too much logic in there, along with the main().

• move out the controller to a ./pkg/...
• move out the router setup to a ./pkg/...
  • possibly splitting out the groups of routes in to functions like API, Omaha, etc
• get the ./cmd/nebraska/ down to a thin veil of parsing flags and called Execute

This will make testing nicer and easier, also will make it possible for clients to import and use shared router for calling the API

[abraarsyed]

Hey, I would love to take up this issue and work on it.

P.S.: I am new to Nebraska project. I worked on a good first issue before and now I am keen on working on this issue. Do you think I can get this up?