feat(api): add langchain agent approach (#5)

[sinedied]

Note: to be rebased or merged for keeping individual commits as steps (some are useful for feedbacks)

[anfibiacreativa]

certainly confusing file names....Is this a convention?

[sinedied]

Yeah it's not the best, it's basically "<chat_or_ask_api>-<approach_name>" but confusing for the "ask" ones.
Initially I wanted to put them in their own folders like chat/read-retrieve-read.ts and ask/read-retrieve-read.ts, but then it would be the class names that would be confusing 😅
If you have a better idea... (naming is even worse on the Python sample BTW)

[anfibiacreativa]

Wouldn't then (if we do / maybe ApiName be a super class to be extended?

like class Chat {}

class ReadRetrieve extends Chat {
           constructor () {
                  super();
           }
}

Or they share no commonalities at all?

[sinedied]

They already implement either the ChatApproach or AskApproach interface (in approach.ts) and all derive from BaseApproach class.
But for example it would result in having two ReadRetrieveRead class to be imported in the plugins/approaches.ts file (so we'll have to discriminate anyways), and having identical class name is not good for searchability of the code :/

I'll try to think of something!