Bleh. Didn't realize until just now that the map method we're using for HTTP endpoints won't work in the (likely common) scenario of the same URI being used for multiple HTTP verbs. Thinking up a fix since the API for Resource Groups (intelligently) uses the same URI for GET and PUT.

This PR now also proposes a fix for the duplicate URI issue described in the comment above. I'm instead using the actual Echo Route class/struct instead of abstracting out into a map. Works with duplicate URIs, removes some boilerplate code from cerulean.go, and allows future endpoints to use all Echo functions if necessary (e.g. middleware).

@goshlanguage I'm not sold on the models package I added as part of this PR (it only contains CloudError currently). I think we're gonna end up using CloudError in other services than just resourcegroups though so we probably want it abstracted. Thoughts on better package/directory naming?

@goshlanguage I'm kinda stumped on why my unit test in /internal/services/resourcegroups/handlers_test.go isn't working. It's not passing the :subscriptionID and :resourceGroupName URI params through to the new handlers I've defined. Might need to just step away from it for a bit and come back, but if you're feeling kind and want to help debug, I wouldn't be opposed. <3