[extension/headersetter] Add support for chaining with other auth extensions

[kushalShukla-web]

Description

This PR adds support for chaining the headersetter extension with other authentication extensions (e.g., OAuth2).
It introduces a new configuration parameter additional_auth in headersetter and ensures that the additional auth extension is started first using the Dependent interface.

Link to tracking issue

Fixes #43935

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[VihasMakwana]

cc: @eduardscaueru

[VihasMakwana]

@kushalShukla-web Can you run make gotidy and commit the changes?

[kushalShukla-web]

@kushalShukla-web Can you run make gotidy and commit the changes?

I ran make gotidy and also tried go mod tidy manually. No changes were produced, and the working tree is clean. Could you confirm if there’s any specific Go version or environment requirement?

[kushalShukla-web]

@VihasMakwana shall we merge this pr ?

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