Cleaned up connection setup by wrapping it with factory method

[oskardudycz]

Fixes #53

Now, it should be easier to implement new ones.

Added also useful helpers that should make the Dumbo usable even as a standalone too. You can do the following:

import { dumbo, rawSql } from '@event-driven-io/dumbo';

const pool = dumbo({ connectionString });

const result = await pool.execute.query<User>(rawSql('SELECT * from users'));

or with transactions:

import { dumbo, sql } from '@event-driven-io/dumbo';

const userId = 32;
const userName = "test@test.com";
const pool = dumbo({ connectionString });

await pool.withTransaction(async ({ execute }) => {
  await execute.command(sql('INSERT INTO users VALUES(%s, %L)', userId, userName));
  await execute.command(sql('INSERT INTO user_roles VALUES(%s, %L)', userId, 'admin') );
})

You can now also share the connection and transaction between your tool and Pongo, e.g. by doing:

import { dumbo, sql } from '@event-driven-io/dumbo';

const userId = 32;
const userName = "test@test.com";
const pool = dumbo({ connectionString });

await pool.withTransaction(async ({ execute }) => {
  // sharing transaction connection with pongo
  const pongo = pongoClient(connectionString, { connection });
  
  const users = pongo.db().collection<User>('users');
  await users.insertOne({ name: randomUUID() });
  await users.insertOne({ name: randomUUID() });

  // doing also insert outside of pongo
  await execute.command(sql('INSERT INTO user_roles VALUES(%s, %L)', userId, 'admin') );
})