This is very much unsafe as the string_ref could point on a stack
variable of the callee.

Original PR #3074 by @dgquintas

- We use C++ as an example to show how this API can be used while still
  providing an idiomatic interface in the wrapped language of choice.
- No testing yet.

- Removing service_accounts credentials. These credentials just have
  drawbacks compared to service_account_jwt_access credentials, notably
  in terms for security.
- Renaming Google specific credentials with a Google prefix for C and
  C++. This should be done as well for wrapped languages.

Add a GrpcLibrary class to wrap grpc_init and grpc_shutdown and convert all the tests to init/shutdown free.

- Renaming default credentials -> google default credentials.
- Various other things in cpp:
  - Adding Cpp wrapping for JWT Tokens.
  - Renaming ComposeCredentials -> CompositeCredentials.

- Remove CredentialsFactory as it's unnecessary
- Effectively make Credentials a channel factory, allowing different credential types to create different channel types - this gives us a hook so that InsecureCredentials can at runtime instantiate a different kind of channel as required - giving us a way of generating an openssl free version of grpc++.
- Server credentials not touched yet, but they'll need to be updated.