[TTS] Add script for mapping speaker names to indices
# What does this PR do ?

The existing TTS workflow requires the user to provide speaker as an integer index in the training manifest. This is awkward because

1. User can provide raw ID from datasets like LibriTTS as an integer, but then we end up creating 10,000+ element speaker array for only hundreds of speakers.
2. User can manually create a map from speaker ID string to sequential integer index and then overwrite their training manifest.
3. Training with multiple manifests is awkward because you need to rewrite the speaker field for all manifests.

So the idea is for the new data loading logic, user will provide speaker as a string name/ID in the manifest and then provide a separate JSON file which maps the speaker IDs to integer IDs. The below script is an example of how to create that map.

**Collection**: [TTS]

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**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation