Support unverified plugins for CLAP node providers

[mfisher31]

The background "unverified plugins" scan (UnverifiedPlugins in src/pluginmanager.cpp) handles JUCE plugin formats but skips Element node providers such as CLAP. The provider loop is currently a stub:

// Element Node Providers
auto& factory = pluginManager.getNodeFactory();
for (auto provider : factory.providers())
{
    // FIXME: CLAP and other unverified support.
}

As a result, CLAP (and other node-provider) plugins never appear in the unverified-plugins list. This should be implemented to mirror the JUCE-format branch: merge in the provider's defaultSearchPath(), run provider->findTypes(...), and store the results.

See src/pluginmanager.cpp:598-603.