[Epic] Improve & Modularize Preprocessing

[tholor]

Let's improve the preprocessing part in Haystack as it has substantial impact on the final accuracy and speed ...

Main Subtasks

1. Restructure "indexing module" (Rename and restructure modules  #379)
2. Let file converters return dicts including meta data (Refactor FileConverters to return dicts incl metadata  #380)
3. Introduce Preprocessor class that does splitting and cleaning (Introduce Preprocessor class that does splitting and cleaning  #381)
4. Improve splitting and cleaning functionalities (e.g. splits that respect sentence boundaries ...) (Better splitting and cleaning functions #382)

[tholor]

Finished all subtasks