feat(python): add storage_options to IvfModel and PqModel save/load

[hushengquan]

Closes #6311

What

Add an optional storage_options keyword argument to IvfModel.save(), IvfModel.load(), PqModel.save(), and PqModel.load(), and forward it to the underlying LanceFileWriter / LanceFileReader.

Why

These methods accept cloud storage URIs (e.g. s3://, gs://) but previously had no way to pass credentials or backend-specific configuration. Users were forced to rely on environment variables for authentication, which breaks down when dealing with multiple storage backends that require different credentials.

Both LanceFileWriter and LanceFileReader already support storage_options — this change simply threads the parameter through.

Change Summary

• python/python/lance/indices/ivf.py: Add storage_options parameter to IvfModel.save() and IvfModel.load(), forward to LanceFileWriter / LanceFileReader.
• python/python/lance/indices/pq.py: Same change for PqModel.save() and PqModel.load().

Usage

from lance.indices.ivf import IvfModel
from lance.indices.pq import PqModel

opts = {"aws_access_key_id": "...", "aws_secret_access_key": "...", "region": "us-east-1"}

# Save
ivf_model.save("s3://bucket/ivf.lance", storage_options=opts)
pq_model.save("s3://bucket/pq.lance", storage_options=opts)

# Load
ivf_model = IvfModel.load("s3://bucket/ivf.lance", storage_options=opts)
pq_model = PqModel.load("s3://bucket/pq.lance", storage_options=opts)

[github-actions]

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[Xuanwo]

Thank you!