[LlamaTokenizerFast] nit update `post_processor` on the fly
# What does this PR do?
This PR adresses #23833 where it appears that being able to change the `add_eos_token` and `add_bos_token` should be made possible for an easier use of the interface. 
Fix comes in two changes 
- added `_add_bos_token` attribute, as well as setters and getters for `add_bos_token`
- added a `self.update_post_processor` that update the post processor based on the current values of `add_eos_token` and `add_bos_token`. 

- added a test to make sure this is properly working
_The documentation is not available anymore as the PR was closed or merged._<|||||>Failing test is unrelated to the changes