feat: support ingest mode for external blob writes

[Xuanwo]

This adds external_blob_mode="ingest" for blob v2 writes so datasets can import external blob bytes into Lance-managed storage instead of always persisting URI references. The write path now streams external sources through lance-io reader streams for packed and dedicated blobs while preserving inline materialization for small payloads.

Closes #6321.

[codecov]

Codecov Report

❌ Patch coverage is 84.77842% with 79 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/blob.rs	89.93%	12 Missing and 19 partials ⚠️
rust/lance-io/src/traits.rs	21.05%	15 Missing ⚠️
rust/lance/src/dataset/write.rs	78.26%	15 Missing ⚠️
rust/lance-io/src/local.rs	53.84%	12 Missing ⚠️
rust/lance-io/src/object_reader.rs	91.42%	2 Missing and 4 partials ⚠️

📢 Thoughts on this report? Let us know!

[wjones127]

It looks like there's a few Windows and linting issues to fix, but otherwise looks good