AWS credential plugin maintenance

[tyrannosaurus-becks]

I am about to start working on #5844 , and I noticed that this plugin could use some updates.

This PR places functions that create paths paths on the backend rather than passing the backend into them. This makes sense because a) they're only used by the backend, and b) a path is something the backend has, so it jives with OOP. We also do it elsewhere now as the standard practice.

This also strips redundant field type declarations like the &Object below, which is no longer preferred in Go:

map[string]*Object {
    "first_thing": &Object{"something"},
}

This also replaces deprecated fields. Vault's implementation of OpenAPI has deprecated the Callbacks field in favor of Operations. AWS has deprecated session.New (which doesn't return an error) in favor of session.NewSession.

I also fixed a couple of minor linting issues along the way.

[tyrannosaurus-becks]

Build failure appears unrelated.

[jasonodonnell]

Great improvements, LGTM! 👍