Fix event SourceFilterSettingsChanged not firing

[ruggi99]

Description

This PR fixes #1303. I've added subscription to update event for filters

Motivation and Context

This issue exists because filters are private by design.
https://github.com/obsproject/obs-studio/blob/30d3b89b3bd02c40ca55d071b2fd038a1c40caa6/libobs/obs-source.c#L436-L460

And, always by design, global/core signals are not fired for private sources
https://github.com/obsproject/obs-studio/blob/30d3b89b3bd02c40ca55d071b2fd038a1c40caa6/libobs/obs-internal.h#L1031-L1042

The solution is to subscribe to source(filter)'s signal update in order to fire the corresponding SourceFilterSettingsChanged event on websocket

How Has This Been Tested?

This PR has not been tested yet. I don't have knowledge to compile this code, only to understading a little bit.
Will test the artifact produced by CI

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.