Marshal/unmarshal complex config as JSON.

[pgavlin]

Terraform providers accept complex (i.e. non-string) config. As it
stands, we require that all provider configuration values are strings,
which causes an unfortunate inability to use these complex configuration
values. These changes enable the use of complex configuration by
serializing and deserializing complex config values using JSON.

Note that this creates a compat issue for future changes to enable
actual rich configuration in the RPC interface; such a future change
will need to accept rich values or strings for top-level properties.

Fixes #48.

[pgavlin]

When using first-class providers, this looks nice:

import * as pulumi from "@pulumi/pulumi";
import * as aws from "@pulumi/aws";

const testing = new aws.Provider("testing", {
    region: aws.config.region,
    assumeRole: {
        roleArn: "<redacted>",
    },
});

const staging = new aws.Provider("staging", {
    region: aws.config.region,
    assumeRole: {
        roleArn: "<redacted>",
    },
});

new aws.s3.Bucket("testing-bucket", {}, { provider: testing });
new aws.s3.Bucket("staging-bucket", {}, { provider: staging });

When using the config system, this is a bit uglier, but it works:

$ pulumi config set aws:assumeRole "{ \"roleArn\": \"<redacted>\" }"

[pgavlin]

(this will fix pulumi/pulumi-aws#252 when rolled out to pulumi-aws)

Ack, wrong issue--this will not fix that.

[lukehoban]

LGTM