Add `key` type and duplicates verification with hashing
Closes #2230 
There is currently no verification for the data type and the uniqueness of the keys yielded by the `dataset_builder`.
This PR is currently a work in progress with the following goals:

-  [x] Adding `hash_salt` to `ArrowWriter` so that the keys belonging to different splits have different hash
-  [x] Add `key` arrtibute to `ArrowWriter.write()` for hashing
-  [x] Add a hashing class which takes an input key of certain type (`str`/`int`/anything convertible to string) and produces a 128-bit hash using `hashlib.md5`
-  [x] Creating a function giving a custom error message when non-unique keys are found 
   **[This will take care of type-checking for keys]**
-  [x] Checking for duplicate keys in `writer.write()` for each batch

[**NOTE**: This PR is currently concerned with `GeneratorBasedBuilder` only, for simplification. A subsequent PR will be made in future for `ArrowBasedBuilder`]

@lhoestq Thank you for the feedback. It would be great to have your guidance on this!