feat: add runtime per-base store params for dataset reads

[Xuanwo]

This adds runtime-only per-base object store params for dataset reads, keyed by BasePath.path, so callers can open different registered bases with different storage options without extending BasePath in the manifest format. The builder now normalizes per-base storage options into ObjectStoreParams once at load time, and Dataset::object_store_for_base uses those params when resolving stores.

This follows the current direction from discussion #6307 to keep provider-specific configuration in runtime state instead of the table format.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/builder.rs	80.00%	10 Missing and 1 partial ⚠️

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[westonpace]

This seems to be ok as is but I have a few questions.

Is it possible to set an option for the default base? Or would you need to override that option in every base?
Can you mask an option to hide it from the bases? For example, maybe the default storage options has a proxy_url but you don't want to use that in the non-default bases?