Add ability to send application data with WebSocket ping frames

[undostres321]

Closes #4593

What?

This PR adds support for sending WebSocket ping frames with application data, as specified in RFC 6455 section 5.5.2. The implementation updates both:

• Standard WebSocket implementation (k6/ws)
• Experimental WebSocket implementation (k6/experimental/websockets)

The WebSocket ping() method now accepts an optional string parameter for application data. Examples:

// Send a regular ping without application data
socket.ping();

// Send a ping with application data
socket.ping("application-data");

Why?

RFC 6455 section 5.5.2 states that ping frames MAY include application data. This implementation:

• Enhances the WebSocket API to fully comply with the WebSocket protocol specification
• Allows users to leverage ping frames for custom application-level keep-alive mechanisms
• Maintains backward compatibility with existing code while adding new functionality
• Properly handles both RTT metrics tracking and application data separation

Checklist

• I have performed a self-review of my code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added tests for my changes.
• I have run linter and tests locally (make check) and all pass.

Checklist: Documentation (only for k6 maintainers and if relevant)

Please do not merge this PR until the following items are filled out.

• I have added the correct milestone and labels to the PR.
• I have updated the release notes: link
• I have updated or added an issue to the k6-documentation: grafana/k6-docs#NUMBER if applicable
• I have updated or added an issue to the TypeScript definitions: grafana/k6-DefinitelyTyped#NUMBER if applicable

Related PR(s)/Issue(s)

Issue #4593: Add ability to ping send Application data per RFC 6455