refactor(js/plugins): implement plugin wrapper that make it easier to work with plugins directly

[pavelgj]

BEFORE:

import { openAI } from "@genkit-ai/compat-oai/openai";
import { type ModelAction } from "genkit/model";

const oai = openAI({ apiKey: "..." });
const gpt4o = (await oai.resolve!("model", "gpt-4o")) as ModelAction;
if (!gpt4o) {
  throw new Error(`failed to resolve gpt-4o`);
}
const response = await gpt4o({ messages: [ ... ] });

AFTER:

import { openAI } from "@genkit-ai/compat-oai/openai";

const oai = openAI({ apiKey: "..." });
const gpt4o = await oai.model("gpt-4o"); // will throw if unable to resolve
const response = await gpt4o({ messages: [ ... ] });

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)