Allow extending other swaggers

[Lindsor]

When using an integration like better-auth, even though they support OpenAPI specs it doesn't get added to the swagger output of Elysia. One workaround could be to allow the swagger plugin to be extended with other swagger definitions so we can add any custom paths to the single swagger definition.

import swagger from '@elysiajs/swagger';

import { auth } from './better-auth/auth';

const betterAuthSwagger = await auth.api.generateOpenAPISchema();

const app = new Elysia()
  .use(swagger({
    extends: [
      betterAuthSwagger,
    ],
  }));

[emirefek]

Also curious about same thing. ElysiaJS have official docs about how to integrate better-auth to elysia but their docs are not showed on swagger. Could we improve that situation?