protobuf 30.x changes various APIs to return string_view's rather than
string&'s. This broke protobuf-c, since we were relying on the implicit
construction of strings from string&'s in various places.

This commit explicitly constructs strings from the string_view's
returned by the protobuf 30.x API when we need to store an owned string
(e.g. in a hash map). This maintains compatibility with older versions
of protobuf such as 3.21 in Debian/Ubuntu while extending compatibility
to the latest protobuf 30.x.

Without this commit we get lots of errors like this when compiling
against protobuf 30.x:

    protoc-gen-c/c_field.cc:119:43: error: no match for 'operator=' (operand types are 'std::map<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >::mapped_type' {aka 'std::__cxx11::basic_string<char>'} and 'google::protobuf::internal::DescriptorStringView' {aka 'absl::debian7::string_view'})