[VitTrack Evaluation] OTB100 Dataset Implementation

[ryan1288]

This PR implements the evaluation script for VitTrack using the OTB100 dataset, as part of an issue raised in https://github.com/opencv/opencv_zoo/issues/119. Reference was made to https://github.com/opencv/opencv_zoo/pull/205 during the development process.

• demo.cpp: Calculates "AUC", "precision", and "normalized precision" using the dataset GT.
• eval.py: Referenced VitTrack's onnx model and OTB as the tracking evaluation dataset.
• __init__.py: Added OTB dataset.
• README.md: Updated with instructions for dataset preparation and evaluation. Simplification from https://github.com/opencv/opencv_zoo/pull/205 ensures complete automation of dataset preparation post-download.

Testing:

• [x] Executed data preparation and evaluation tests.

1. python eval.py -m vittrack -d otb -dr /path/to/otb
2. Within the tools/eval directory, python eval.py -m vittrack -d otb -dr .

Both procedures yield identical results regardless of the presence of the OTB100.zip file, the correct placement of OTB.json, or the preprocessing status of the folders.