libobs: Add functions to serialize JSON with default values

[derrod]

Description

Adds two new functions to serialize obs_data objects to JSON while also including default values.

Motivation and Context

Alternative to #10641 that requires opt-in from the API user. Same motiviation and context.

How Has This Been Tested?

Serialized some data.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

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

[Lain-B]

Would recommend documentation (if documentation for this area of functions already exist) but not a big deal one way or another imho