Add tables/models for zoekt enabled namespaces and indices

What does this MR do and why?

Related to #424121 (closed)

This MR implements the schema design defined here. The plan is to replace Zoekt::IndexedNamespace class and zoekt_indexed_namespaces table with two new classes/tables Search::Zoekt::EnabledNamespace (zoekt_enabled_namespaces and Search::Zoekt::Index (zoekt_indices)

This MR is broken up into multiple iterations

MR	What it does
⭐ !138992 (merged)	Add database migrations and model classes
!140252 (merged)	Migrate data and use new models
TBD	Drop old tables and remove code

Screenshots or screen recordings

N/A

How to set up and validate locally

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

MR acceptance checklist

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