Allow adding plugins via configuration

[kaklakariada]

Is your feature request related to a problem? Please describe.

OFT can only discover third-party plugins from the fixed directory
$HOME/.oft/plugins/<plugin-name>/*.jar. There is no public API to tell OFT about
plugin JARs at runtime:

• ServiceLoaderFactory is package-private and hardcodes the plugin directory.
• InitializingServiceLoader.load(Class<T>, C) is the only public entry point and
  accepts no plugin locations.
• ClassPathServiceLoader.filterOtherClassLoader rejects any service whose
  getClass().getClassLoader() differs from the origin's class loader.

This forces embedders that resolve plugins through their own dependency mechanism
(e.g. the Gradle plugin openfasttrace-gradle, which resolves pluginDependencies
via a Gradle configuration) to work around the API by replacing the thread context
class loader (TCCL) with a child-first loader. Because of the class-loader identity
filter above, that workaround must additionally locate and copy OFT's built-in
provider JARs (importer/exporter/reporter factories) into the same child loader —
otherwise built-in reporting and importing silently disappear as soon as any plugin
is configured.

The result is a fragile, hard-to-maintain workaround in the embedder that duplicates
logic OFT already implements correctly for $HOME/.oft/plugins (ServiceOrigin +
ChildFirstClassLoader).

Describe the solution you'd like

A public, programmatic way to pass plugin JAR paths to OFT, e.g.:

Oft oft = new OftRunner(List.of(pluginJar1, pluginJar2));