Add module to load/dump standard protocol buffers

[popematt]

Issue #, if available:

None

Description of changes:

Adds a module that provides functionality to make it easier to support standard protocol buffers in the benchmark CLI. Functionality includes:

• Wrapper for a protocol buffer Message that provides load/loads and dump/dumps for that message type
• Function to dynamically load a protocol buffer Message from a python module at runtime
• Function to dynamically generate a protocol buffer Message from a proto file descriptor at runtime

The functions for loading the Message are important because we can't hard-code the Message implementation into the benchmark CLI (since it's not necessarily known at compile time), so we need some way to load them at runtime.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.