[feature] Add option to use localhost as redirect host for oauth2

[nyalldawson]

Some oauth2 implementations (eg the MS Sharepoint implementation) do not support http://127.0.0.1 redirects. They allow only https://127.0.0.1 OR http://localhost redirects.

Since https is not simple to support, instead expose an option to allow localhost for redirects.

[nyalldawson]

@elpaso look ok to you?

[qgis-bot]

@nyalldawson

This pull request has been tagged for the changelog.

• The description will be harvested so please provide a "nearly-ready" text for the final changelog
• If possible, add a nice illustration of the feature. Only the first one in the description will be harvested (GIF accepted as well)
• If you can, it's better to give credits to your sponsor, see below for different formats.

You can edit the description.

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[qgis-bot]

@nyalldawson
This pull request has been tagged as requiring documentation.

A documentation ticket will be opened at https://github.com/qgis/QGIS-Documentation when this PR is merged.

Please update the description (not the comments) with helpful description and screenshot to help the work from documentors.
Also, any commit having [needs-doc] or [Needs Documentation] in will see its message pushed to the issue, so please be as verbose as you can.

Thank you!

[github-actions]

@nyalldawson
A documentation ticket has been opened at qgis/QGIS-Documentation#8671
It is your responsibility to visit this ticket and add as much detail as possible for the documentation team to correctly document this change.
Thank you!

[elpaso]

LGTM