Add Pass User Identities entry to "User Preferences" table

What does this MR do and why?

This MR creates a new entry in the User preferences table. This entry pass_user_identities_to_ci_jwt will eventually be toggled via API to include the entire list of the user's external identities in the JWT information sent to the job.

See the relevant issue here: #387537 (closed)

Title	Status
Add toggle for enabling user_identities to database/models	👈 You are here
Add user_identities field to JwtV2
Add API for changing the toggle
Document the new field

Screenshots or screen recordings

No UI changes are needed.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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