Add transaction functionality in the same way serverless-mysql does

[GuthrieW]

Why?

• I needed this functionality and ended up extending it within my own code, I would imagine others would appreciate this functionality being part of the library.
• As this package is inspired by serverless-mysql, it seems to make sense to try to support the same functionality that it has.

Desired Changes

• Support transactions using the same methods as serverless-mysql: https://www.npmjs.com/package/serverless-mysql#transaction-support

[GuthrieW]

@MatteoGioioso it seems I'm not able to push branches to the repo. The required code change would not be very large, one function and the associated tests. I'd be happy to do the rest of the work for this. If I can't be given permissions, I'm happy to just leave the code here:

ServerlessClient.prototype.transaction = function () {
  const _logger = this._logger
  const query = this.query

  const queries = []
  let rollback = () => {}

  return {
    query: function (query) {
      _logger("Adding query to transaction")
      queries.push(query)
      return this
    },
    rollback: function (func) {
      if (typeof func === 'function') {
        this.rollback = func
      }
      return this;
    },
    commit: async function () {
      const results = []

      try {
        _logger("Beginning transaction")
        await query('BEGIN;')

        for (let i = 0; i < queries.length; i++) {
          _logger(`Executing transaction query ${i}`)
          const result = await query(queries[i])

          _logger(`Transction Query ${i} complete`)
          results.push(result)
        }
    
        _logger('Committing transaction')
        await query('COMMIT;')
    
        _logger('Transcation complete')
        return results
      } catch (error) {
        _logger('Transaction error, rolling back')
        rollback();
        await query('ROLLBACK;')
      }
    },
  }
}

The code is called the same way as serverless-mysql:

 const results = await serverlessPsqlClient.transaction()
  .query('INSERT INTO table (x) VALUES(?)', [1])
  .query('UPDATE table SET x = 1')
  .rollback(e => { /* do something with the error */ }) // optional
  .commit() // execute the queries