Allow customization of the authorization process

[fpoirotte]

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Changelog: (Feature): Allow the authorization process to be customized, just like authentication. This fixes #5758. Docs: https://github.com/conan-io/docs/pull/2684

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• [x] If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• [x] I've read the Contributing guide.
• [x] I've followed the PEP8 style guides for Python code.
• [x] I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

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[fpoirotte]

Hi,

I did a few manual tests yesterday using the example plugin I posted in the docs' PR. My tests worked as I intended and I did not notice any adverse effect.