feat: Merge the Model, Agent and RagAgent types into single Agent type

[cvauclair]

Feature Request

Merge the Model, Agent and RagAgent types into a single Agent type.

Motivation

The Model, Agent and RagAgent types currently all work the same under the hood. The only difference between all of them is the amount of configuration each type supports, from least configurable (Model) to most configurable (RagAgent). In other words, an Agent without defined preamble, context or tools is equivalent to a Model and a RagAgent without defined vector stores is equivalent to an Agent.

Merging those types into a single, highly customizable Agent type would improve the developer experience and reduce code repetition.

Proposal

• Remove the rig::model::Model (the entire rig::model module can probably be removed entirely) and rig::rag::RagAgent types.
• Expand the definition of the rig::agent::Agent type to include the same parameters as the old rig::rag::RagAgent type.
• Expand the rig::agent::AgentBuilder type to reflect the new parameters.
• Move all RAG specific logic which can be reused easily to rig::rag module

Alternatives

Another solution could be to have a RAG system type be parametrized by any other type that implements the Prompt and/or Chat trait (e.g.: and Agent type). However, this would mean that there are two toolsets (one for the RAG system and the other for the underlying Agent) which complicates tool calling.

[0xMochan]

Implemented by #21