Fixes debug logging of CfnResource (#198)

Before this change, logging could generate "not supported" errors in the
stream.

I1104 14:23:22.081768 66128 eventsink.go:59] CfnResource
aws-native:ec2:SubnetRouteTableAssociation:
{"routeTableId":"Calling [toJSON] on an [Output<T>] is not
supported.\n\nTo get the value of an Output as a JSON value
or JSON string consider either:\n 1: o.apply(v => v.toJSON())\n 2:
o.apply(v => JSON.stringify(v))\n\nSee
https://www.pulumi.com/docs/concepts/inputs-outputs for more
details.\nThis function may throw in a future version of
@pulumi/pulumi.","subnetId":"Calling [toJSON] on an [Output<T>] is not
supported.\n\nTo get the value of an Output as a
JSON value or JSON string consider either:\n 1: o.apply(v =>
v.toJSON())\n 2: o.apply(v =>
JSON.stringify(v))\n\nSee
https://www.pulumi.com/docs/concepts/inputs-outputs for more
details.\nThis function may throw
in a future version of @pulumi/pulumi."},
["awsId","routeTableId","subnetId"]

After the change, debug logging should happen only after the outputs are
resolved.

There is a slight downside for unknowns - debug logging will not happen
when some of the properties are unknown anymore, because this will make
`pulumi.output(x)` unknown and will not call `debug` in `apply`.

Note also that running programs under PULUMI_DEBUG_GRPC="$PWD/log.json"
environment variable should also make it possible to observe how the
underlying resources are constructed in terms of calls to the backing
provider, including observing unknowns.

src/interop.ts

@@ -94,7 +94,9 @@ export class CfnResource extends pulumi.CustomResource {
     ) {
         const resourceName = typeToken(type);
 
-        debug(`CfnResource ${resourceName}: ${JSON.stringify(properties)}, ${JSON.stringify(attributes)}`);
+        debug(`Constructing CfnResource ${name} of type ${resourceName} with attributes=${JSON.stringify(attributes)}`);
+        const propertiesDebugString = pulumi.output(properties).apply(JSON.stringify);
+        pulumi.interpolate`CfnResource ${name} input properties: ${propertiesDebugString}`.apply(debug);
 
         // Prepare an args bag with placeholders for output attributes.
         const args: any = {};