Clean up shared module

[kevinwallimann]

Currently, not all classes in module shared are really used in multiple modules. With #99 the module needs to be published to maven. To keep its footprint small, only classes whose responsibilities span across multiple modules should be kept in shared. All other classes should be scoped package private on za.co.absa.hyperdrive.

Tasks

• ConfigurationKeys: Move to ingestor-default package, only move object IngestorKeys to driver module.
• IngestionException and IngestionStartException: Move to driver module
• FileUtils, ConfigUtils should be moved to ingestor-default package. Also merge it with the SchemaRegistrySettingsUtil from Add KafkaWriter #108 . They should be made package private on za.co.absa.hyperdrive
• SparkUtils should be removed since it has no usages. With it, also TestSparkUtils should be removed. Then, the dependency on testutils can be removed. testutils can be removed altogether.

[kevinwallimann]

• ConfigurationKeys: Will be refactored with Refactor ConfigurationKeys #117
• IngestionException: Won't move
• SparkUtils: Has been removed