libobs/util: Add buffered file serializer

[derrod]

Description

Adds a buffered file serializer, essentially porting the feature added in #5717 to the libobs utilities and serializer API.

Motivation and Context

See #5717 for the reasons of wanting a buffered writer.

As the existing one is only in the ffmpeg muxer binary, but I also needed it for the native MP4 muxer, I figured it'd be good to make it available via the generic OBS serializer API so it can be reused in other components if it makes sense to do so. For example, we may consider also using it in the native FLV output.

Split out from #10608 for easier review and slimming down that PR as things get merged.

How Has This Been Tested?

Tested as part of #10608.

Types of changes

• New feature (non-breaking change which adds 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.

[RytoEX]

Looks okay at a glance. Left some questions/nits.

[RytoEX]

Looks okay at a glance now.