Improve how secrets are handled in Dynamic Providers docs

[lukehoban]

Docs like https://www.pulumi.com/docs/concepts/resources/dynamic-providers/#example-github-labels-rest-api use an approach to manage secrets with dynamic providers which doesn't work well in practice. We've added improved support for capturing Secret values into dynamic providers reently, and should adopt those into these examples, and also likely add explicit documentation about how secrets can be used with Dynamic Providers.

[justinvp]

We should update the Node.js example along the lines of https://github.com/pulumi/pulumi/blob/master/tests/examples/dynamic-provider/secrets/index.ts from https://github.com/pulumi/pulumi/pull/13329

And we need to get https://github.com/pulumi/pulumi/pull/15864 merged, and then update the Python example similarly.