Add CSV source

[emgrav]

Blocked by #49 (This PR adds a source trait which we should implement for a CSV source)

Description

We want to add an additional data source to the sync tool, specifically CSV (or some other form of flatfile source). We have a need for this quite soon, as it would significantly simplify the process of bootstrapping the demo environment for Operations and Marketing.

User story

As a Famedly customer, I want to be able to provide a csv file of user details to import into Famedly so I can easily provision my list of users in bulk.

Requirements

• Read CSV file on disk in format: email,first_name,last_name,phone
• Creation of new users in the list. No support for updating or deleting users at this point.

Steps to implement

• Add a new implementation of the source trait that reads a CSV file from disk