Make the configuration for an artifact a struct (using a well defined validating function)

[cgruber]

Right now, the config is an arbitrary dictionary. Making it a struct (or built through a function with well defined parameters) can allow it to be a bit safer. Instead of:

"foo.bar:baz:1.0": {
    "sha256": "abcdef...",
    "build_snippet": SOME_SNIPPET,
}

You'd do

"foo.bar:baz:1.0": config(
    sha256 = "abcdef...",
    build_snippet = SOME_SNIPPET,
),

Using this would allow us to change the underlying mechanism for holding the data and communicating it, immunizing the user from those changes, as well as having more obvious errors earlier (can't call that function with an unknown param, for instance).