Fix sensitive client_secret config specification

[fanny-jiang]

What does this PR do?

Fix the client_secret config option to mark the field as sensitive.

Motivation

QA https://github.com/DataDog/integrations-core/pull/12891

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[codecov[bot]]

Codecov Report

Merging #12983 (9fdb4b2) into master (0b92164) will decrease coverage by 0.00%. The diff coverage is n/a.

Flag	Coverage Δ
kafka_consumer	84.03% <ø> (-0.26%)	:arrow_down:

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