Add snapshot support to Data Connector service

### Required Terms

- [X] I agree to follow this project's [Code of Conduct](https://github.com/heroku/roadmap/blob/main/CODE_OF_CONDUCT.md)
- [X] I have read and accept the [Salesforce Program Agreement](https://www.salesforce.com/company/program-agreement/)

### What service(s) is this request for?

Heroku Streaming Data Connector

### Tell us about what you're trying to solve. What challenges are you facing?

We were hoping to use Heroku's Streaming Data Connector service to maintain a separate copy of a database. Our use case requires a full copy of the database, but the Data Connector service unfortunately does not have a way to snapshot existing data.

How hard would it be for Heroku's Streaming Data Connector service to support taking an initial snapshot before streaming new data?