Allow alternative schemes in Binder

[tomurb]

Description

Allows for adding before_parse hooks to the Binder#parse method.

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• [x] I have added (or updated) appropriate tests if this PR fixes a bug or adds a feature.
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• tests went thorugh on my fork: https://github.com/tomurb/puma/actions

Closes #3302

[dentarg]

It would be good to document / show how one can use this (it is not entirely clear to me, even when I've read the linked issue, but I have also never developed a plugin for Puma)

Can something be added to https://github.com/puma/puma/blob/master/docs/plugins.md?

[tomurb]

I've added some documentation. I also didn't develop any plugins. I don't know how to write an example. I could maybe add that link for context and add how that change would make difference for acme-puma codebase? (change of the method name, and removal of lib/puma/acme/binder.rb file)

Does that sound right?

[dentarg]

could maybe add that link for context and add how that change would make difference for acme-puma codebase?

I think that would be good