uri: rework struct uri to use a copy of the original splitted string.

In the future, it is planned to extend the URI structure to allow its
passing with different options and in different formats (see next commit
`uri: implement ability to parse URI query paramters` for example). For
these purposes, it is planned to use functions that modify the source
string, for example `strtok_r`, so we need to rework the URI structure
to create copies of the string for each of the URI components.

Part of #5928