Migrations for WebAuthn

What does this MR do?

This is yet another preparation MR for !26692 (merged), extracting the migrations into a separate MR to split the code change into several smaller MRs (as suggested here).

It introduces a new table webauthn_registrations which will later contain the saved webauthn registrations, adds a foreign key from this table to the user table and adds a webauthn_xid to the user details table. For details, please see the main MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides (I hope!)
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done