Sync eng/common directory with azure-sdk-tools for PR 12758 (Azure#43911)

* add reference doc for adding client customizations in TypeSpec

* Update eng/common/knowledge/customizing-client-tsp.md

Co-authored-by: Copilot <[email protected]>

* Apply suggestions from code review

Co-authored-by: Mariana Rios Flores <[email protected]>

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Co-authored-by: Christopher Radek <[email protected]>
Co-authored-by: Christopher Radek <[email protected]>
Co-authored-by: Copilot <[email protected]>
Co-authored-by: Mariana Rios Flores <[email protected]>

Author: 5 people