Add an assistant builder

[SommerEngineering]

Once we have established the specifications for assistant plugins, we will offer an assistant builder. Initially, this idea was referred to as "RAG Bundles" #24. Independently of RAG, the assistant builder will enable users to create their own assistants without any programming. Plugins will be described using the Lua language, see #14. Accordingly, the builder will generate a Lua script. Thus, it is possible to offer assistants without data and independently of any particular LLM, similar to the assistants currently provided by AI Studio.

Additionally, an assistant can be tightly connected to an external ERI data source and a retrieval method. Furthermore, an assistant can specify the LLM provider as well as the model. This is useful because some assistants may want or need access to specific model capabilities.