## Summary

The Security Solution team has test cases that intentionally use
`esClient` directly without `apiClient`, so enforcing the
`scout_require_api_client_in_api_test` rule on their scout tests
produces false positives. Instead of excluding Security Solution from
the rule entirely, this PR enhances the rule with a configurable
`alternativeFixtures` schema option so it can accept additional fixtures
per directory.

## Changes

- Added `alternativeFixtures` schema option to
`scout_require_api_client_in_api_test`, allowing directories to
configure additional accepted fixtures via `.eslintrc.js`
- Generalized `functionUsesApiClient` to `functionUsesFixture`,
parameterizing the fixture name so the same detection logic works for
any fixture
- Error messages are dynamic and list only the accepted fixtures for the
given configuration
- Added a Security Solution override in `.eslintrc.js` with
`alternativeFixtures: ['esClient']` so their tests accept either
`apiClient` or `esClient`
- Added unit tests for the new `alternativeFixtures` option

---------

Co-authored-by: Dzmitry Lemechko <dzmitry.lemechko@elastic.co>
(cherry picked from commit 24d327b)

# Conflicts:
#	x-pack/solutions/security/plugins/entity_store/test/scout/api/tests/dsl_translation.spec.ts
#	x-pack/solutions/security/plugins/entity_store/test/scout/api/tests/esql_translation.spec.ts
#	x-pack/solutions/security/plugins/entity_store/test/scout/api/tests/painless_translation.spec.ts