rpcclient: add ListLabels RPC using RawRequest

[studentyashsrivastava]

Change Description

Adds a ListLabels() helper to rpcclient using RawRequest.

This allows callers to access the listlabels RPC without modifying or
removing the existing account-based helpers and avoids introducing any
breaking changes.

This follows the RawRequest approach discussed in #1974.

Steps to Test

Build the project:
go build ./...

Run all tests:
go test ./...

Pull Request Checklist

Testing

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