Add plugin system licensed as LGPLv3

[Measurity]

Describe the issue

We're open to contributions to help develop this system as a separate component (.NET assembly) under the Nitrox Organization that will be licensed as LGPLv3. The plugin system will have to work without calling functions on any GPLv3 code, which is doable by going in opposite direction (Nitrox calling the new plugin system). The dependency chain looks like:

Nitrox (GPLv3) -> Generic Multiplayer Mod Plugin Specification (LGPLv3) <- External Plugin/Mod (LGPLv3-compatible)

The new plugin system should be a versioned interface API, allowing updates without breaking older plugins.