Resolve "Surface code suggestion in User Preference API"

What does this MR do and why?

Adds the user preference code_completion to the public user preferences API

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. GET /api/v4/user/preferences and notice code_suggestions is available
2. PUT /api/v4/user/preferences to modify code_suggestions

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.

Related to #425710 (closed)