Dynamically-generate static files

**Is your feature request related to a problem? Please describe.**

ZeroNet only supports serving static site files which can add dynamic functionalities with client-side JavaScript. However, there are some cases where client-side generated files are not possible. Some of these cases can be command line browsers, programmatic HTTP requests, usage of ZeroNet as package index for PyPI, NPM, Packagist, Docker...

**Describe the solution you'd like**

There should be support for files that are generated on the backend (like normal client-server architecture). This could be made using BackgroundProcessing plugin, which would be listed on specific URLs (routes) and return dynamically-generated content (like modern MVC frameworks).

@HelloZeroNet @imachug What do you think?

Start by reviewing the proposed BackgroundProcessing plugin and the specific URL routes that would return generated content. Define the supported route behavior and generated-file cases before implementation; the issue does not name files or tests.

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