[Fix] Overhaul slipstream plugin setup/framework

[dgrkotsonis]

Slipstream plugin manager: static-only architecture with inventory self-registration

Motivation

The original implementation used #[no_mangle] C FFI to load plugins as .so/.dylib files at runtime via libloading. We've since reevaluated that need, as Rust's lack of a stable ABI makes dynamic loading of trait objects an inherent production risk, and the decoupling benefit it offered is still mostly preserved. Static linking with inventory self-registration is the cleaner and safer approach.

Changes Made

• Added PluginRegistration { name, factory } + inventory::collect! to the plugin interface. Plugin crates self-register at link time via inventory::submit!, eliminating a need for hardcoded discovery logic in snarkOS
• Removed all dynamic loading code from the plugin manager: LoadedPlugin, load_plugin(), from_config_files(),
  resolve_libpath_from_config(), load_plugin_from_config(), the Library::from(this()) shim, and the libloading/json5 dependencies
• register() is now the only plugin path; Drop for LoadedSlipstreamPlugin calls on_unload() directly