feat(api): field shared_subscriptions in endpoint /monitor_current

[JimMoen]

Fixes https://emqx.atlassian.net/browse/EMQX-11994

Release version: v/e5.6.0

Summary

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Added property-based tests for code which performs user input validation
• Changed lines covered in coverage report
• Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• [ ] If changed package build workflow, pass this action (manual trigger)
• [ ] Change log has been added to changes/ dir for user-facing artifacts update