Enable manifest 'headers' to be a function and to be interpreted

Currently, a manifest's 'path' object and individual 'params' properties can be either an object or function.

A use case came up where it would be desirable for an individual header property to be generated from a function (an API key retrieved from a plugin's config needed to be passed as a header in a request to an external API).

We need to provide the ability for headers to be able to use text substitution and also be generated via a function.