Support for Headers

Thanks for a very useful plugin!

Had a thought concerning this: https://github.com/google/strict-csp/blob/main/strict-csp/index.ts#L116

Perhaps headers could be supported as well. Maybe there could be some configuration setting whether to generate and inject the meta tag or perhaps just generate a file containing the hashes. The generated file could then be consumed elsewhere e.g. Server middleware in order to inject the hashes into a CSP.

Just a thought, that would side-step the problems raised in the link above (CSP not protecting before the meta tag + no support for report-only mode).

Thought I'd raise the topic for discussion, happy to submit a PR if in any interest. Feel free to resolve immediately if you feel there's no mileage in it.