[b/398251186] Make descriptions mandatory for Snowflake connectors

[misolt]

Changes

• Make it required for non-abstract Snowflake connectors to provide a description string.
• Other existing connectors are unaffected.

Implementation

Add the method getDescription:

• The method is abstract in AbstractSnowflakeConnector
• In AbstractConnector, the implementation uses the existing getAnnotation approach
• In Connector, the method returns an empty description.

Notes

• The result is that all AbstractSnowflakeConnector subclasses need to provide a description string.
• In addition, providing an empty description is no longer possible - it will be hidden. Let me know if you intended to use it and I'll add support.
• It is now possible (but not that useful) to build more complex description strings, because they don't have to be constants.
• Implementing a Connector that bypasses AbstractConnector and then putting the Description annotation on it would've worked in the past, but it will not work after this change.