Introduce more robust HTTP methods (with rich body and response types)

[bitspittle]

See https://docs.google.com/document/d/1xaAgpPJ-qVE0zJ-J0G9Yswsx6XnoT2KuIjZgzXSuwBA/ for more details

This will be a two-step bug. For the first step, introduce new bytes versions of each method (e.g. get -> getBytes, post -> postBytes) and aggressively push users to migrate over. Users will get deprecation warnings that will be easy to migrate over.

In the second step, introduce new versions of the old methods that take a Body instead of a ByteArray and return a Response instead of a ByteArray, breaking backwards incompatibility.