New training data format (Initilal Implementation)

[degiz]

Umbrella ticket for the initial implementation of the New Training Data Format.

The latest format examples are in the notion.

We can logically split the work into three parts:

1. NLU: needs new YAML parser + support separation of lookup tables. Issue: New Training Data Format - NLU #5983
2. Stories: needs new YAML parser + support Rules. New Training Data Format - Stories #5996
3. Domain: add support to split the file. Issue: New Training Data Format - Domain #5984

For the initial implementation IMO it's enough to implement only read functionality for the parsers, dump can be done later.

NLU and Stories are still using --data parameter, and --domain should accept the folder name.

Later we can decide is we want to allow the keys from domain (i.g. intents) to be mixed with for example NLU data.

Later we should also implement a converter from the existing format to the new one.

To a certain amount the work can be parallelized, so every point above can be implemented separately.