Add UI control to switch between Zoekt and Elasticsearch

What does this MR do and why?

Related to #384726 (closed)

Add UI control to switch between Zoekt and Elasticsearch

query plan: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/19267/commands/63578

Click to expand

 Limit  (cost=17.59..19.14 rows=1 width=4) (actual time=18.948..18.954 rows=0 loops=1)
   Buffers: shared hit=3 read=10
   I/O Timings: read=18.677 write=0.000
   ->  Merge Join  (cost=17.59..20.69 rows=2 width=4) (actual time=18.946..18.952 rows=0 loops=1)
         Merge Cond: (zoekt_indexed_namespaces.namespace_id = namespaces.id)
         Buffers: shared hit=3 read=10
         I/O Timings: read=18.677 write=0.000
         ->  Index Only Scan using index_zoekt_indexed_namespaces_on_namespace_id on public.zoekt_indexed_namespaces  (cost=0.13..3.17 rows=3 width=8) (actual time=2.694..2.695 rows=1 loops=1)
               Heap Fetches: 0
               Buffers: shared read=2
               I/O Timings: read=2.642 write=0.000
         ->  Unique  (cost=17.46..17.47 rows=2 width=4) (actual time=16.248..16.252 rows=0 loops=1)
               Buffers: shared hit=3 read=8
               I/O Timings: read=16.034 write=0.000
               ->  Sort  (cost=17.46..17.46 rows=2 width=4) (actual time=16.247..16.251 rows=0 loops=1)
                     Sort Key: namespaces.id
                     Sort Method: quicksort  Memory: 25kB
                     Buffers: shared hit=3 read=8
                     I/O Timings: read=16.034 write=0.000
                     ->  Append  (cost=0.56..17.45 rows=2 width=4) (actual time=16.180..16.184 rows=0 loops=1)
                           Buffers: shared read=8
                           I/O Timings: read=16.034 write=0.000
                           ->  Index Scan using index_namespaces_on_owner_id on public.namespaces  (cost=0.56..3.58 rows=1 width=4) (actual time=7.523..7.523 rows=0 loops=1)
                                 Index Cond: (namespaces.owner_id = 10)
                                 Filter: ((namespaces.type)::text = 'User'::text)
                                 Rows Removed by Filter: 0
                                 Buffers: shared read=4
                                 I/O Timings: read=7.468 write=0.000
                           ->  Nested Loop  (cost=1.00..13.83 rows=1 width=4) (actual time=8.653..8.655 rows=0 loops=1)
                                 Buffers: shared read=4
                                 I/O Timings: read=8.566 write=0.000
                                 ->  Index Scan using idx_members_on_user_and_source_and_source_type_and_member_role on public.members  (cost=0.56..10.38 rows=1 width=4) (actual time=8.651..8.651 rows=0 loops=1)
                                       Index Cond: ((members.user_id = 10) AND ((members.source_type)::text = 'Namespace'::text))
                                       Filter: ((members.requested_at IS NULL) AND (members.access_level >= 10) AND ((members.type)::text = 'GroupMember'::text) AND (members.access_level = ANY ('{20,30,40,50}'::integer[])))
                                       Rows Removed by Filter: 0
                                       Buffers: shared read=4
                                       I/O Timings: read=8.566 write=0.000
                                 ->  Index Only Scan using index_groups_on_parent_id_id on public.namespaces namespaces_1  (cost=0.43..3.45 rows=1 width=4) (actual time=0.000..0.000 rows=0 loops=0)
                                       Index Cond: ((namespaces_1.parent_id IS NULL) AND (namespaces_1.id = members.source_id))
                                       Heap Fetches: 0
                                       I/O Timings: read=0.000 write=0.000

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Before	After

How to set up and validate locally

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MR acceptance checklist

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