Support streaming blobs for published messages

V5.0.0 limits blob usage inside published messages, always embedding the blobs. This is done so that transaction logs can be safely purged by deleting files without requiring the transaction log be scanned for blob references that need to be removed. However, currently only blobs created from a buffer can be used in published messages, streaming blobs won't work. We should add support for streaming blobs.

The issue names no files, tests, or entry points. Start by tracing how published messages handle buffer-created blobs and compare that path with streaming blobs; done means streaming blobs can be used in published messages without breaking the existing blob and transaction-log behavior.

Written by the indexing model from the issue text.