Add support for creating instances of resources in Python using a
`<Resource>Args` class. This capability aligns with how args are passed
to resources in all the other language SDKs and the separate object bag
allows the properties to be manipulated/validated/passed-around before
creating the resource.
We've been emitting calls to `New<Resource>` for resource registrations
in Go, passing `nil` for args. However, some of those `New<Resource>`
functions actually check for `nil` args and return an error if the
resource has required arguments.
At first, I was looking for a way to check inside `New<Resource>` if
the `URN` option was specified and in that case not error on
`nil` args (like we do in other languages), but we don't provide a way
to access the resource option values outside the Go SDK), so I don't
think there is a way to do it this way for Go.
So instead, this change updates the registration code to call
`ctx.RegisterResource` directly instead of `New<Resource>`, where we can
pass a `nil` args.
These changes extend the type reference parser in the schema package to
accept references of the form "(package/version/schema.json)?#/provider".
These references refer to the package's provider type, which is
otherwise not referenceable, as it is not present in the "resources"
array.
