Removed grpc_security.h from auth_context.h

Regenerated doxygen after #3227

Replicate C++ docs from master to beta branch

Original PR #3074 by @dgquintas

Refactor Endpoint API

Respond with UNIMPLEMENTED when a requested method is not found

Fixed routeguide namespace, updated README to match other changes

Fix bugs that were causing auth interop tests to fail

PHP: bump version

Fix C# route_guide example

Give interop tests more time to build

PHP: support per message compression disable

C# update examples to proto3

Fix C# nuget package building

It may seem weird that code and details would travel along two paths
now instead of one but it makes sense after considering that sometimes
the code and details are application data from the remote application
and sometimes they are transport data from the transport between the
local and remote applications.

Expose per-call credentials to Python.

PHP: fix jenkins macos build error