Add RestSharp.Serializers.CsvHelper

[ishanpranav]

Description

I recently started using RestSharp to create a C# wrapper library for the AlphaVantage financial data REST API. For memory reasons, Alpha Vantage's more data-intensive APIs output only to CSV (not JSON or XML). All its endpoints support CSV responses, so it makes sense to use CSV as the standard data exchange format for the class library I am writing.

As a result, I added CSV serialization support to RestSharp via the robust CsvHelper library. This pull request includes an adapter between RestSharp and CsvHelper that supports both single-line comma-separated records (individual objects) and full CSV tables (collection types inheriting IEnumerable).

So far, I've added tests and tried to mimic the precedents/conventions set by the other serializer projects, but I'm not sure what documentation needs to be created.

Best,
Ishan Pranav

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)