Ensure identity queries use lower query to match index

What does this MR do?

Fixes #331465 (closed).

Ensures all identities queries use the with_extern_uid scope, which ensures the use of lower. In addition to the github provider identified in the issue, I found similar issues in the bitbucket and gitlab importers. I added a new scope to user and pointed all cases there.

Database index is on:

lower(extern_uid), provider

Database

Before:

SELECT
    "users".*
FROM
    "users"
    INNER JOIN "identities" ON "identities"."user_id" = "users"."id"
WHERE
  "identities"."provider" = 'github'
  AND "identities"."extern_uid" = 'my_github_id' 

After:

SELECT
    "users"."id"
FROM
    "users"
    INNER JOIN "identities" ON "identities"."user_id" = "users"."id"
WHERE (LOWER("identities"."extern_uid") = LOWER('my_github_id'))
    AND "identities"."provider" = 'github'
ORDER BY
    "users"."id" ASC
LIMIT 1

Explain: https://explain.depesz.com/s/TU43

 Limit  (cost=7.04..7.04 rows=1 width=4) (actual time=4.602..4.605 rows=0 loops=1)
   Buffers: shared hit=5 read=2
   I/O Timings: read=4.501 write=0.000
   ->  Sort  (cost=7.04..7.04 rows=1 width=4) (actual time=4.600..4.602 rows=0 loops=1)
         Sort Key: users.id
         Sort Method: quicksort  Memory: 25kB
         Buffers: shared hit=5 read=2
         I/O Timings: read=4.501 write=0.000
         ->  Nested Loop  (cost=0.99..7.03 rows=1 width=4) (actual time=4.556..4.558 rows=0 loops=1)
               Buffers: shared hit=2 read=2
               I/O Timings: read=4.501 write=0.000
               ->  Index Scan using index_on_identities_lower_extern_uid_and_provider on public.identities  (cost=0.56..3.57 rows=1 width=4) (actual time=4.553..4.554 rows=0 loops=1)
                     Index Cond: ((lower((identities.extern_uid)::text) = 'my_github_id'::text) AND ((identities.provider)::text = 'github'::text))
                     Buffers: shared hit=2 read=2
                     I/O Timings: read=4.501 write=0.000
               ->  Index Only Scan using users_pkey on public.users  (cost=0.43..3.45 rows=1 width=4) (actual time=0.000..0.000 rows=0 loops=0)
                     Index Cond: (users.id = identities.user_id)
                     Heap Fetches: 0
                     I/O Timings: read=0.000 write=0.000

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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Security

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