adding support to encrypt values using GoCD API

[MFAnderson]

This is not ready per se, but want feedback sooner rather than later.

GoCD encrypts secure environment variables so they aren't stored in plain text in the config XML. Since 17.1.0 an API endpoint has existed to allow users to encrypt values client side. By adding this to gomatic we can support users setting secure environment variables from their gomatic config.

[dudadornelles]

What if we actually write an integration test for this one?

[ghost]

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