[Feature Request] Use Regex for attribute mappings

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Feature request

Which Nextcloud Version are you currently using: 28.0.11

Is your feature request related to a problem? Please describe.
We are using parts of the email as user name in Nextcloud. It is impossible to extract and map them from our IdP's fields.

Describe the solution you'd like
I would like to choose in the attribute mappings of provider settings, if i want to run the input through a regex.
It could also be used to extract a domain or other infos.

Describe alternatives you've considered
Either the IdP can provide this info in a custom field in the OID profile (our doesn't).
Or another request to the IdP to get more user infos, which would be an OAuth request in our case.

Additional context
UI mockup:

Start with the provider settings attribute-mapping flow and review the linked UI mockup. Clarify the regex syntax, extraction behavior, validation, and matching rules before locating the implementation. Done means regex-based transformation or extraction is available for attribute mappings while existing mappings continue to work.