feat: add pro backend constants

[Aerilym]

No description provided.

[Copilot]

Pull Request Overview

This PR adds support for Session Pro backend constants by exposing payment provider metadata and URLs through the Node.js API. The changes integrate new constants from the libsession-util library and make them accessible to JavaScript code.

Key changes:

• Added conversion support for string8 type to JavaScript strings
• Exposed Session Pro URLs (roadmap, privacy policy, terms of service, support URL, etc.)
• Exposed payment provider metadata for Nil, Google Play Store, and iOS App Store platforms

Reviewed Changes

Copilot reviewed 3 out of 5 changed files in this pull request and generated 2 comments.

File	Description
src/constants.cpp	Added new static constants for Pro URLs and payment provider metadata, mapping C structures to JavaScript objects
libsession-util	Updated submodule commit reference to include Pro backend support
include/utilities.hpp	Added toJs_impl specialization for string8 type conversion

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