Replace const string& by string_view

[bernardnormier]

We should replace const string& by string_view in most or all Ice APIs.

It's possible that in some cases const string& should be replaced by string--when the implementation adopts the string.

For example: https://github.com/zeroc-ice/ice/blob/eee150539a2c4bdad0e8043f911aa3a49e936a70/cpp/include/Ice/Proxy.h#L327 https://github.com/zeroc-ice/ice/blob/eee150539a2c4bdad0e8043f911aa3a49e936a70/cpp/include/Ice/Communicator.h#L154 https://github.com/zeroc-ice/ice/blob/eee150539a2c4bdad0e8043f911aa3a49e936a70/cpp/include/Ice/ObjectAdapter.h#L268