[Plugins] General purpose plugin system

[alvrs]

• We need the core of MUD to be as small and as possible to make it easier to understand and easier to maintain, and eventually audit
• At the same time we want to be able to support arbitrary complex use cases

→ We need a rock solid plugin system!

Plugins can:

• modify the MUD config + types
• modify the codegen / build / deploy pipeline
• provide client libraries to interact with whatever the plugin provides
• provide systems / tables / modules to add contract functionality

[alvrs]

part of #1668 / #1561