feat: extractor api improvements

[0xMochan]

• [x] I have looked for existing issues (including closed) about this

Feature Request

• Extractors exist as a wrapper on top of agents with a pre-made tool. Since it constructs and abstracts the agent away, this means you can't customize the agent. This is a problem for anthropic as anthropic requires max_tokens to be set atm. This is also quite restrictive in other regards.

• Extractors are also missing the prompt trait in general making them less composible (as per the feat/agentic-chains branch).

• Extractors only work with full types and not with any serde_json::Value which means things like Vec<MyType>, bool, etc won't work.

• Lastly, extractors are built with the use of tools to produce a json output but with the release of json-mode and similar in many providers (like OpenAI, etc), it might be suitable for provider specific or customizable extractors that make use of that feature rather than a custom tool. This customizability isn't really hit within our extractor implementation.

Proposal

Rather than Extractor being a whole struct, I wonder if .extract::<Type>(...) would be a better api. Or some other Extractor trait based on the provider that can be written (otherwise using a default tool implementation) to make this a bit more seemless. It seems like a general extractor improvements branch is in order.

Alternatives

Keeping the Extractor struct but making the agent exposable or even configurable. All the bugs could be fixed in the current implementation.