CXX driver should allow a way to update a `uri` similar to how the C driver allows.  Scenario: start from a C-string that is like: `
mongodb://localhost:27017/Play` and want to set the `AppName` after parsing the URI.  The C driver supports this scenario but the CXX driver `uri` class does not offer setters.
As an alternative, if the C driver API supported converting `mongoc_uri_t` into a string, the building up/modification of the uri could be done with the C API and then converted into a C string and used as the input for the CXX driver.