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

• [✔] Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

Code Style and Documentation

• [✔] The change is not insubstantial. Typo fixes are not accepted to fight bot spam.
• [ ✔] The change obeys the Code Documentation and Commenting guidelines, and lines wrap at 80.
• [ ✔] Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.

📝 Please see our Contribution Guidelines for further guidance.

[studentyashsrivastava]

any suggestions on what you need since its not reviewed for long time?