Harmonize module structure

[chuwy]

This is an umbrella ticket for work I've done on restructuring the code of S3 Loader and does not bring any user-facing changes. Main goals were:

1. Extract everything related to kinesis-connector into its own connector module to make it easier to factor out later
2. Remove several unused entities from previous versions such as Thrift decoders
3. Harmonize configuration: remove stringly-typed properties, use circe codecs, move everything configuration-related into one module, add a test spec, provide better error messages
4. Merge KinesisS3Emitter and S3Emitter - they're the same thing