libbrillo: Blob combining utility Add a function that concatenates given Blob's. This provides analogous functionality of SecureBlob::Combine() to Blob. BUG=chromium:728047 TEST=new unit test (BlobTest.CombineBlobs) Change-Id: I33218bd81a49a265d021e7afdcf438b7fc70921d Reviewed-on: https://chromium-review.googlesource.com/1089338 Commit-Ready: ChromeOS CL Exonerator Bot <[email protected]> Tested-by: Maksim Ivanov <[email protected]> Reviewed-by: Jorge Lucangeli Obes <[email protected]>