Add ability to get a list of supported pipeline tasks
Author: codesue
Pull request: yes

# What does this PR do?

This change adds the ability to programmatically get a list of supported pipeline tasks.

Currently, when new users want to know which tasks are supported in a pipeline, we have to reference the [Pipelines documentation](https://huggingface.co/docs/transformers/main_classes/pipelines) or parse the SUPPORTED_TASKS and TASK_ALIASES dictionaries. Being able to  programmatically get a list of supported pipeline tasks would be more convenient since it requires less context switching and parsing.

Example usage:
```
>>> from transformers.pipelines import get_supported_tasks
>>> print(*get_supported_tasks(), sep=“\n”)
audio-classification
automatic-speech-recognition
conversational
feature-extraction
fill-mask
image-classification
image-segmentation
ner
object-detection
question-answering
sentiment-analysis
summarization
table-question-answering
text-classification
text-generation
text2text-generation
token-classification
translation
zero-shot-classification
```

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Comments:
Thanks for the PR! 

Off-topic from the actual PR but for background information, this is where the "canonical" list of pipelines (even outside of transformers) supported by the HuggingFace hub is stored: https://github.com/huggingface/huggingface_hub/blob/main/widgets/src/lib/interfaces/Types.ts

Thanks for the reviews, everyone! And, wow, that was so fast! 🤩