DPR - enable loading of custom BERT models

[Timoeller]

Add proper loading and saving of DPR models. It also supports other Bert models (e.g. with different vocab)

• We want to use processor.save and model.save for saving and load those models in the same way.
• A test is added for saving and loading the processor and model
• Adds shuffling of DPR data right after loading the file

Code tested in haystack as well, needs adjustments to loading, but simplifies code a lot.

[julian-risch]

Tested on my side with gbert. Works like a charm.