[TTS] Add script for text preprocessing
# What does this PR do ?

A lot of people already have a local script which does text processing & normalization before training. To do it quickly one needs to use multiprocessing or joblib, as our text normalizer does not support batch processing.

It only does normalization and optional lower casing of the final output right now, but could be extended with additional processing functionality as needed.

**Collection**: [TTS]

# Changelog 
- Create text preprocessing script
- Add example config file for English text normalization

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation
