libobs: Add experimental API to retrieve next audio timestamp on source

[norihiro]

Description

This PR adds a new API obs_source_get_next_audio_timestamp, which returns the timestamp struct obs_source::next_audio_ts_min.

This PR will make the PR below obsolete.

• libobs: Implement asynchronous sample rate conversion for audio #6351

Motivation and Context

The timetamp indicates the timestamp that the the next call of obs_source_output_audio should provide, assuming if there is no delay in the audio filters.

This should be helpful for my plugin async-audio-filter. The usage will be like this PR. Without this API, the plugin need to estimate next_audio_ts_min by a complicated code and still it is not always accurate.

I originally hoped the PR #6351 but the PR looks stale. Instead, I suggest to just add an API and let the plugin to do the similar job of #6351.

How Has This Been Tested?

OS: Fedora 39

Tested with the modified plugin.

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 are squashed where appropriate.
• I have included updates to all appropriate documentation.

[RytoEX]

I believe the proposal during triage was that this should be marked as deprecated/experimental to indicate that it is a stopgap solution until a more complete solution can be produced. cc @tt2468

[norihiro]

Added OBS_DEPRECATED to the function declaration and mentioned it is experimental and deprecated in the document as below.

.. deprecated:: experimental
   This is a stopgap solution until a complete solution is implemented.