Add Silence Augmentation
Signed-off-by: fayejf <fayejf07@gmail.com>

# What does this PR do ?

Add support for online silence perturbation for data augmentation  and robustness test. 

**Collection**: ASR

# Changelog 
- Add silence at the beginning and/or end of an utterance. 

# Usage
* You can potentially add a usage example below

```python
train_ds:
    .....
    augmentor:
      silence:
        prob: 0.8
        min_start_silence_secs: 0
        max_start_silence_secs: 5
        min_end_silence_secs: 0
        max_end_silence_secs: 5
```

# 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)
- [x] Did you write any new necessary tests?
- [ ] Did you add or update any necessary documentation?
       Will add this to [Online_Noise_Augmentation tutorial](https://github.com/NVIDIA/NeMo/blob/main/tutorials/asr/Online_Noise_Augmentation.ipynb) later if necessary
- [ ] 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

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
