mac-au: Add an AUv3 host as filter

[pkviet]

Description

This adds an AUv3 host to obs-studio.
The host is loaded as a filter.
It is compatible with both v2 and v3 AudioUnits which provide audio filters on macOS.
Sidechain is supported.
Resizing of the AU GUI is also supported (that was tricky).
The AU host follows version 3 specifications, which add a bridge allowing to load AU version 2.

Some screenshots:
AUv2 from Apple

AUv3

Sidechain

Sidechain and GUI

Motivation and Context

AU filters are native to macOS. They're a very good alternative to VST on macOS.
macOS ships with a series of good AU filters.

I coded a VST3 host and decided to look at the competition (AU & CLAP).
It seemed easy to write an AU host so for fun, I did it. The architecture is the same as that of obs-vst3.

How Has This Been Tested?

Tested on macOS 26.1.
AU both v2 & v3 work fine, with or without GUI.
Tested the GUIs resize fine.

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.