This release introduces google::cloud::functions::Function, an opaque wrapper
to represent any C++ function to handle HTTP or CloudEvent requests. Application
developers can use this wrapper (via the google::cloud::functions::MakeFunction()
factory functions) to capture the signature of their functions. Soon, the
GCP buildpacks will no longer require GOOGLE_FUNCTION_SIGNATURE_TYPE,
which can be error-prone.

• feat: support functions::Function in Run() (#340)
• feat: make pkg-config files relocatable (#336)
• doc: how-to guide on using legacy code (#334)