inputs: Add audio mono requests and event

[aniolpages]

Description

Adds GetInputAudioMono, SetInputAudioMono, ToggleInputAudioMono, and InputAudioMonoChanged.

Motivation and Context

Exposes OBS Studio's per-input mono setting through obs-websocket.

How Has This Been Tested?

Built against OBS Studio master and tested all requests and the event with stereo and mono media inputs, using both input names and UUIDs.

Tested OS(s): macOS 26.6 (x86_64)

Types of changes

• New request/event (non-breaking)

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.