Part of aligning HttpDataSource behavior will require adding
logic that's common across the DataSource implementations. This
change establishes a util class to house it, and moves a bit of
existing logic that's related and can be easily shared into it.
There is one small behavior change in this CL, which is that our
handling of Content-Range response headers can now parse the body
length if the "document size" part of the Content-Range is unknown,
for example "bytes 5-9/*". Previously the pattern we were matching
to required the "size" part to be set, for example "bytes 5-9/100",
despite the fact we don't need or use it.
PiperOrigin-RevId: 362396976
