Synthetics - add ability to update existing checks and download .side file via configsource

With synthetic transactions, currently you need to delete/recreate a check if we want to update it or change the username/password used for authentication. This causes loss of historical data and is difficult to manage. There are 2 things I would like to request that would make it easier for us to manage synthetic transactions.

1. add ability to update an existing check. This includes updating the .side file or updating the authentication
2. Add ability to download the .side file for an existing check via a companion Configsource. This would make it easier to resolve issues with a check and tweak it without impacting historical data.