Add truncation_side option to tokenizers
# What does this PR do?

As requested by #12909, it would be handy if one could also decide on whether to truncate sequences from the left instead of from the right. 

As we already have a `padding_side` (which can be either left/right), it makes sense to also add a `truncation_side` (which by default is set to `"right"`, but users can initialize a tokenizer with `truncation_side` set to `"left"`).

The test could possibly be improved (for which I'd like to get some help).

Also requesting review from @patrickvonplaten since I've also added the option in `feature_extraction_sequence_utils.py`. 

Regarding the fast tokenizers, I see `padding_side` is used [here](https://github.com/huggingface/transformers/blob/12e02e339f6d19218b36a30883188ea0254bc7e7/src/transformers/tokenization_utils_fast.py#L362). Should I define something similar for `truncation_side`?

Fixes #12909 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>~Gentle ping @NielsRogge for when you have a chance to wrap this up~ Issue opened in tokenizers :)<|||||>It is waited for more than a year now (https://github.com/huggingface/transformers/issues/4476#issuecomment-677823688). Please implement this (even if it works out just for ordinary tokenizers), for people to use now this solution, while users of Rust tokenizers wait for the fast tokenizers solution (https://github.com/huggingface/tokenizers/issues/779).<|||||>Fixed per #14947.