libobs, frontend: Add functions to save json with defaults

[cg2121]

Description

This saves the default OBS data. With this, we don't have to create new versions of objects when we want to change the default settings.

Motivation and Context

Continuation of #10693

How Has This Been Tested?

Created a new source, made sure I didn't change any settings, and made sure the settings were saved to the json file.

Types of changes

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

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[cg2121]

I've noticed this has already been added with #10693, it was just not used when saving OBS. I've updated this PR to add save json with default functions.