fix: Get registration fields from site configuration

[DmytroAlipov]

Description

This is a backport from the master branch

There is currently no way to control the REGISTRATION_EXTRA_FIELDS setting in Site Configuration.

This fix adds the ability to control whether or not this field is displayed on the legacy registration page.

By default, data is taken from the settings.

Changes in the admin panel do not affect the behavior of this field in the Authn MFE:

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[DmytroAlipov]

Something is broken in the pipelines. Can anyone help with this?

[mphilbrick211]

@e0d is this something you can help with?

Same question on #33165

[e0d]

Rerunning the check seems to have resolved it.

[DmytroAlipov]

@mphilbrick211 @e0d Thanks 😊

[mphilbrick211]

Hi @openedx/vanguards! Would someone be able to please review and merge for us after the master is merged? Thank you!

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