Add SAML provider search to UsersFinder

What does this MR do?

Adds the by_saml_provider_id query param to UsersFinder.

• This was done so that lib/api/users.rb can later be changed (in a separate MR: !66167 (merged)) to accept this param too, so that get /users can be queried by the Invite members modal, to display the suggested users to invite list, when the group or project's group is using enforced group_saml.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/333625

Required by !63565 (merged) and !66167 (merged)

There is no changelog entry because it is developer-facing only and nothing is using the new finder param yet. The API will use this in the next MR, and the changelog and documentation will go with that one.

SQL

SELECT "users".* FROM "users" INNER JOIN "identities" ON "identities"."user_id" = "users"."id" WHERE "identities"."saml_provider_id" = 1

QUERY PLAN

                                                QUERY PLAN
-------------------------------------------------------------------------------------------------------------
 Nested Loop  (cost=0.27..9.67 rows=2 width=1387)
   ->  Index Scan using index_identities_on_saml_provider_id on identities  (cost=0.12..4.16 rows=3 width=4)
         Index Cond: (saml_provider_id = 1)
   ->  Index Scan using users_pkey on users  (cost=0.14..1.83 rows=1 width=1387)
         Index Cond: (id = identities.user_id)
(5 rows)

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
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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)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team