[client] Support specifying rack when adding/removing server tags in Admin API

[LiebingYu]

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Description

Task Overview:

Currently, Admin.addServerTag() and Admin.removeServerTag() only support specifying tablet servers by their server IDs. This task introduces two new overloaded methods that allow users to target all tablet servers belonging to a specific rack in a single call.

Purpose:

In production environments, the rack attribute of a ServerNode typically represents a physical machine (or a physical rack of machines). A common operational scenario is decommissioning a physical host — which may run multiple tablet servers — and marking all of them as PERMANENT_OFFLINE or TEMPORARY_OFFLINE at once.

Without rack-based targeting, operators must:

1. Call Admin.getServerNodes() to retrieve all server nodes.
2. Manually filter the results by the target rack.
3. Extract the server IDs.
4. Call addServerTag() / removeServerTag() with those IDs.

This is repetitive, error-prone, and adds unnecessary boilerplate in automation scripts. A dedicated rack-based API makes the decommissioning workflow concise and intuitive.

Willingness to contribute

• I'm willing to submit a PR!