Include additional field in session data

import { betterAuth } from "better-auth";
import { drizzleAdapter } from "better-auth/adapters/drizzle";
import { db } from "./db/index";
import * as schema from "./db/schema";

export const auth = betterAuth({
  database: drizzleAdapter(db, {
    provider: "pg",
    schema,
  }),
  user: {
    fields: {
      name: "display_name",
    },
    additionalFields: {
      username: {
        type: "string",
        defaultValue: null,
        input: false,
        unique: true,
      },
    },
  },
  socialProviders: {
    google: {
      prompt: "select_account",
      clientId: process.env.GOOGLE_CLIENT_ID!,
      clientSecret: process.env.GOOGLE_CLIENT_SECRET!,
    },
  },
});