Remove opaqueness for grpc_cl_connection:connection(), make it accessible from outside

[cabol]

It is useful to have access to connection attributes, for example, suppose you want to monitor the Pid of the connection, in that case we need to access the http_connection attribute.

Now, if the purpose is encapsulate it, other option would be provide getters functions within the grpc_cl_connection module, like:

http_connection(#{http_connection := Value}) -> Value.

host(#{host := Value}) -> Value.

scheme(#{scheme := Value}) -> Value.