NFA subtitle file config - specify colors and vertical alignment
# What does this PR do ?

Modify the `ASSFileConfig` so we can specify the colors of the text and its vertical alignment.

**Collection**: `tools/nemo_forced_aligner`

# Changelog 
For specifying vertical alignment, removed `ass_file_config.marginv` parameter, which was being used as a rough way to modify the vertical placement of the text. Now, there is a parameter `ass_file_config.vertical_alignment`, which can take the values `"center"` (default), `"top"`, or `"bottom"`

For specifying the colors of the text, added 3 new parameters:
- `ass_file_config.text_already_spoken_rgb`
- `ass_file_config.text_being_spoken_rgb`
- `ass_file_config.text_not_yet_spoken_rgb`

# Usage
Can specify new parameters in `ASSFileConfig` e.g.:

```bash 
python <path to NeMo>/tools/nemo_forced_aligner/align.py \
  pretrained_name="stt_en_fastconformer_ctc_large" \
  manifest_filepath="manifest.json" \
  output_dir=nfa_output/ \
  additional_segment_grouping_separator="|" \
  ass_file_config.text_already_spoken_rgb=[252,186,3] \ # bright yellow/orange
  ass_file_config.text_being_spoken_rgb=[240,26,240] \ # bright pink
  ass_file_config.text_not_yet_spoken_rgb=[62,26,240] \ # bright blue
  ass_file_config.vertical_alignment="top"
```

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

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# Additional Information
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