MIDI FX AudioUnits can retrieve their current sample rate from an output
bus. However, it's reasonable for a JUCE MIDI FX plugin to require 0
output buses, in which case the correct sample rate cannot be retrieved.

This patch modifies the AU wrapper to ensure that MIDI FX plugins will
always have at least one output bus. This bus does not necessarily
correspond to a matching bus in the AudioProcessor, and it will not be
passed to AudioProcessor::processBlock. With the output bus in place,
the correct sample rate is reported in `prepareToPlay`.

This patch also moves PluginHostType into juce_audio_processors in order
to allow hiding of function implementations.

Reports the correct host name for Apple hosts on M1 which host plugins
in a separate process.

Affected hosts are:
- Logic Pro
- GarageBand
- MainStage
- Final Cut Pro

Previously, the `JUCE_APP_CONFIG_HEADER` macro wasn't being defined/included
in some of the wrapper files, which meant that there was a danger of these
files building without important JucePlugin_Build_* macros defined.