cpp: model BDE bdlbb::Blob byte-buffer taint flow

Add flow summaries for the BDE segmented byte buffer
BloombergLP::bdlbb::Blob so taint reaches a blob's payload bytes:

- Accessor chain: Blob::buffer taints the returned BlobBuffer, and
  BlobBuffer::data/buffer taint the bytes.
- bdlbb::BlobUtil::copy and getContiguousRangeOrCopy propagate taint
  between a blob and a flat buffer in both directions.

This unblocks blob-carried sources such as bmqa::Message::getData, whose
payload was previously stranded on the opaque Blob object. Not a
duplicate; the bdlbb namespace had no coverage. Verified with a
BloombergLP::bdlbb-shaped stub in the dataflow external-models harness.