Consider XML support for object PUT/GET

[coryan]

First, thank you for writing this!

Some client libraries (e.g. C++) automatically switch to XML for some uploads and downloads. You may want to consider supporting the XML APIs to insert and download objects. In case it helps, our testbench implements these. These are the entry points, hopefully the code is not so terrible that you can follow it from there:

https://github.com/googleapis/google-cloud-cpp/blob/abc8e9c3e95c561924a8ae567012440152d097b6/google/cloud/storage/testbench/testbench.py#L50-L67