Lv2 support work

[saintmatthieu]

Resolves: audacity/audacity#8302

Two things:

• Added Lv2Plugin to enum class AudioResourceType ; MSS may or may not support LV2s in the future, but the muse::audio::AudioResourceMeta, used for the plugin registration, has a AudioResourceType field.
• Added virtual audio::AudioResourceType metaType() const = 0 to muse::audioplugins::IAudioPluginMetaReader ; this supports the implementation in Audacity of a AbstractAudioPluginMetaReader, which basically makes plugin discovery functionality available for reuse between different meta-reader implementations.

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• I created a unit test or vtest to verify the changes I made (if applicable)

QA:

• Plugin registration still works