Improve access to witnesses in BlobTransactionBuilder

[MujkicA]

Currently, if the user adds a blob via BlobTransactionBuilder::with_blob, the corresponding witness will not be available until .build() is called. This may cause confusion when the user wants to add a predicate which relies on the blob witness to the transaction.

We should attempt to mitigate confusion here, options include setting the witness when with_blob() is called or providing a helper similar to UploadTransactionBuilder::prepare_subsection_upload.