Backend work for regex mode toggle for Zoekt code search

What does this MR do and why?

Wrap the search term with double quotes if the param exact_search is passed. This feature is backed by a feature flag zoekt_exact_search with the user actor

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Before	After

How to set up and validate locally

• Setup Zoekt for a namespace
• Make sure the checkbox Enable Zoekt code search under User Preferences is checked
• Enable the feature flag zoekt_exact_search

Feature.enable(:zoekt_exact_search)

• Perform a code search in the chosen namespace.
• Open the zkt section in the performance bar. Ensure the searched term is wrapped with a double quote
• Add a query param regex=true or regex=1 in the searched URL
• Open the zkt section in the performance bar. Ensure the searched term is NOT wrapped with a double quote

Related to #434417 (closed)