Support secrets in config

[blampe]

	test := providertest.NewProviderTest(
                ...
		providertest.WithConfig("password", os.Getenv("PASSWORD")),
	)

If "password" is marked as a secret in config, e.g.

config:
  password:
    type: string
    secret: true

the test will fail with

error: validating stack config: Stack 'p-it-bryces-wor-yaml-b7f19499' with configuration key 'password' must be encrypted as it's secret

This is a good thing, since we don't want to accidentally leak any credentials! But we should probably expose a WithSecret option to allow passing encrypted config.