Fixes for Connector#validate

[misolt]

• Add a method validateForConnector called by AbstractSnowflakeConnector#validate. The method validate handles common checks for the data source, while validateForConnector handles checks for a specific connector, e.g. snowflake-logs. This is done to ensure that a connector doesn't accidentally skip common validation.
• Add Nonnull to the param of validate. We don't expect ConnectorArguments to be null.
• Make validateDateRange non-default. This method is not core connector functionality, it doesn't depend on the connector instance and it shouldn't be overridden. A static method fits better for the usage.
• Simplify some checks in validateDateRange to make it less nested.