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

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[RobertIndie]

Could you help add the unit test for it?

[ericsyh]

Okay, I will add test for new funcs.