[kbn-eslint-plugin-eslint] add scout_api_test_discourage_kbn_client rule

[steliosmavro]

Summary
Adds a new ESLint rule to discourage the use of the kbnClient fixture within API tests in Scout. The rule emits a warning if kbnClient is unpacked or used directly in the test body, guiding test authors to avoid using it for endpoint validation and to follow best practices.

Why

• Prevents misuse of kbnClient in API tests, which can lead to privilege issues or unclear test intent.
• Encourages test authors to use apiClient for endpoint validation, maintaining consistency and clarity in test design.

What changed

• Introduced a new ESLint rule that checks for direct usage or unpacking of kbnClient in API test blocks.
• The rule emits a warning when kbnClient is used, but can be overridden with a comment in rare, justified cases.

How to use

• Write API tests as usual, avoiding direct use of kbnClient in the test body.
• If the rule triggers in a valid edge case, suppress the warning with an ESLint comment and provide a justification.

[cla-checker-service]

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