Data source implementation for Snowflake Data source

[hainv-ohio]

• Data source class implementation: Make sure it has needed properties and fields.
• Unit tests for fields and constructor
• rdash file loading test: write a test to make sure you can load a rdash document sample file (generated by Reveal) that has this DataSource, and assert all properties are loaded correctly
• Dashboard creation Sandbox example: Write sandbox sample code that uses this data source, making sure it can be used to create a Dashboard.
• RdashDocument creation test: From the sandbox sample above, write a test to make sure the data source can be used with the RdashDocument successfully to create a dashboard and assert the key fields are set correctly.
• Json export test: Exported JSON should have the correct fields.
• Write API document: Write as comment doc on top of the class and fields