Provide built-in support for "write-only" fields for secret/hidden values

[emk]

When using an OpenAPI interface to manage passwords, we may want to allow fields like password to appear in the input, but not in the results of GET requests. We could do this by adding a field secret: true, and either:

1. Removing the field entirely from the GET version of a structure, or
2. Allowing it to be present, but only with a placeholder value.

The decision between these two choices is ongoing inside Faraday.