Filter Protobuf nullable warnings

Update ObjC versions for 0.12 release

The podspec is a template.

Cpp new creds plugin api

Changing the credentials plugin API.

Also passing empty string as opposed to NULL when the method name is not
found. This is much less error-prone.

The purpose of this is to be able to install a composition policy that
describes which types are incompatible and that will be enforced during
call creds composition.

If this functionality is wanted it will be done in an additive function in
the API like :

void grpc_call_credentials_set_composite_policy(
  grpc_call_credentials_composite_policy policy);

Updated Node package version to 0.12.0

bump C# version to 0.12.0

- The plugin is now passed more information that it can use to create
  auth metadata:
    - service_url (as before)
    - method name
    - channel_auth_context

Core credentials API refactoring

Fixing NULL terminator for test cert.

Forgot to do that when I replaced the cert in #4096. Thanks asan for
catching that!

Updating the server1 cert so that it can be used with Go.

Merge release-0_11 into master

Handle Cancel before start.

Add missing grace value to server stop in Python example.

Update Python C wrapping layers to new credentials API

Fix dead link in doc