Initial support for remote component construction.

[pgavlin]

These changes add initial support for the construction of remote
components. For now, this support is limited to the NodeJS SDK;
follow-up changes will implement support for the other SDKs.

Remote components are component resources that are constructed and
managed by plugins rather than by Pulumi programs. In this sense, they
are a bit like cloud resources, and are supported by the same
distribution and plugin loading mechanisms and described by the same
schema system.

The construction of a remote component is initiated by a
RegisterResourceRequest with the new remote field set to true.
When the resource monitor receives such a request, it loads the plugin
that implements the component resource and calls the Construct
method added to the resource provider interface as part of these
changes. This method accepts the information necessary to construct the
component and its children: the component's name, type, resource
options, inputs, and input dependencies. It is responsible for
dispatching to the appropriate component factory to create the
component, then returning its URN, resolved output properties, and
output property dependencies. The dependency information is necessary to
support features such as delete-before-replace, which rely on precise
dependency information for custom resources.

These changes also add initial support for more conveniently
implementing resource providers in NodeJS. The interface used to
implement such a provider is similar to the dynamic provider interface
(and may be unified with that interface in the future).

An example of a NodeJS program constructing a remote component resource
also implemented in NodeJS can be found in
tests/construct_component/nodejs.

This is the core of #2430.

[pgavlin]

The tests that are failing in CI are also failing locally, but I'm at a loss as far as which changes would cause the failures.

[justinvp]

Still looking this over, but here are some initial comments.

[justinvp]

LGTM