Make ByteStream_StorageType available via static const method
ByteStream_StorageType is an external variable referenced by all
bytestream converters. Make this available via a static constant method in
ByteStreamAddress to be used by all clients. This is more explicit and
will also allow tools like the thread checker to correctly deduce that
this is a constant value. ByteStreamAddress was chosen as the new home because all
existing converters already have a dependency on that package.
As a first example, migrate TrigHLTResultByteStream.
cc @cranshaw