Ability to pickle/unpickle BatchEncoding pickle (reimport)
Overrides the methods get_state() & set_state() to (respectively) export the content of the underlying data dictionnary and - if defined - the content of encodings.

Unittests added to covert the serialization & deserialization of all the exported properties.

Reimported from #4515
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=h1) Report
> Merging [#5039](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/36434220fc807c5015bc8f0f1e50ab21f7d34914&el=desc) will **increase** coverage by `0.00%`.
> The diff coverage is `100.00%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5039/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #5039   +/-   ##
=======================================
  Coverage   77.36%   77.37%           
=======================================
  Files         130      130           
  Lines       21989    21998    +9     
=======================================
+ Hits        17012    17021    +9     
  Misses       4977     4977           
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/tokenization\_utils\_base.py](https://codecov.io/gh/huggingface/transformers/pull/5039/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fdXRpbHNfYmFzZS5weQ==) | `91.69% <100.00%> (+0.13%)` | :arrow_up: |
| [src/transformers/file\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5039/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9maWxlX3V0aWxzLnB5) | `75.00% <0.00%> (-0.41%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/5039/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `87.42% <0.00%> (+0.15%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=footer). Last update [3643422...5e40fe4](https://codecov.io/gh/huggingface/transformers/pull/5039?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
