Allow customization of the authorization process

[fpoirotte]

Changelog: Feature: Allow the authorization process in conan_server to be customized, just like authentication.
Docs: conan-io/docs#2684

Closes: #5758

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[memsharded]

We were thinking of requesting to add some tests, copying whatever the custom_authentication had. It seems that such tests don't even exist, so I am fine merging this as-is, no need to add tests, I guess that you have already tested this manually, haven't you?

Thanks for contributing to Conan.

[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.