feat: support the namespace offloadThresholdInSeconds API in pulsaradmin pkg

[ericsyh]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

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Motivation

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Modifications

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Verifying this change

• [x] Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

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• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)

Documentation

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[ericsyh]

Okay, I will add test for new funcs.