Add selection criteria for reference audios in the `GlobalStyleToken` submodule
# What does this PR do ?

- Allow specifying the criterion with which the _reference audio_ is selected for a given training/validation example

**Collection**: TTS

# Changelog 
- The current NeMo implementation selects the reference audio for a given training example as a random example from the same speaker. As discussed in #7777, this allows the GST module to extract speaker information, and _not_ speaking style information, from the reference spectrogram, thereby making it easier to fine-tune on new speakers with very little data
- On the other hand, the [GSTs paper](https://arxiv.org/abs/1803.09017) states that:
    > During training, the reference signal is ground-truth audio.
- As discussed in #7777, it is desirable to support both use cases as opposed to just one of the two

# 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


## Who can review?
@subhankar-ghosh, @hsiehjackson


# Additional Information
* Related to #7777
