feat: generate prompts from datasets

[XuehengWang]

1. Generate prompts from datasets, specified in datasets-config-file

• We can load datasets from both local files and Hugging Face datasets.
• Round-robin selection based on each dataset's selection ratio.
• Requests can be randomly shuffled or maintain their original order.
• Added some example configuration files in the datasets_config folder

2. In SyntheticTraceGenerator, add random and Poisson-distributed timestamps

3. Unit test for data_loader

[future-xy]

What is this file for?

[future-xy]

Our repo don't actually have this file?

[XuehengWang]

let me test loading from local files using a different approach :)