Support custom headers in elm client

[hmac]

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Description of the PR

This change adds a new option to the Elm generator: enableCustomHeaders. When enabled, the generated request functions will allow you to set your own headers (e.g. for authorisation). It's very similar to the basePath option. I would really like to make this change without introducing another option or breaking existing code, but Elm's records don't support optional fields or default values, so this is the best I could do.

Very interested to hear feedback if there's a better approach here!

cc @trenneman

[hmac]

Just seen this is a duplicate of #1704 - will close this for now on the assumption that that gets merged.