Dataset Tool to add Timestamps

[farzadab]

This PR adds a TimestampGeneration task to ds_tool.py. It uses Montreal Forced Aligner (MFA).

The task adds a new column (e.g. "timestamps") that has word-level alignments in the form of:

[ {"start": float in seconds, "end": float in seconds, "text": first word str}, ... ]

Usage example:

# Install Conda environment named aligner
# and download english_mfa acoustic model and dictionary
just install_mfa

# Run ds_tool (make sure upload name is not the same as input dataset_name)
just ds_tool timestamp -d fixie-ai/common_voice_17_0 -S en --upload_name fixie-ai/cv_ts  \
    -m english_mfa -T "\"{{text_proc.format_asr_text(sentence)}}\""

[farzadab]

Actually if the changes here are okay I think I'll split ds_tool into multiple files, one per task, since this file is getting a bit too long.
I'll do it in the next PR.