Connectors: Dedicated forms for OLAP engines

[ericpgreen2]

This PR provides an in-app guided flow for connecting to external OLAP engines (currently: ClickHouse, DuckDB, Pinot).

Specifically:

• Reframes the "Add source" modal as the "Add data" modal, and splits it into two sections: Sources and OLAP engines
• Adds forms for connecting to ClickHouse and Pinot
• Compiles the OLAP engine forms into connectors/<connector-name>.yaml files

TODO:

• Platform support for connector.yaml files