net/kea: fix build with libc++ 19

As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.

This causes net/kea to fail to compile with clang 19 and libc++ 19,
resulting in errors similar to:

    In file included from edns.cc:9:
    In file included from ../../../src/lib/exceptions/exceptions.h:11:
    /usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'
      820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
          |                                          ^
    ../../../src/lib/dns/name.h:727:16: note: in instantiation of template class 'std::basic_string<unsigned char>' requested here
      727 |     NameString ndata_;
          |                ^
    /usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
       23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
          |                             ^

`NameString` is defined as `std::basic_string<unsigned char>`, which is
no longer possible. So redefine it as a `std::vector<unsigned char>`
instead.

This requires a few adjustments in other places: adjusting the arguments
for the `assign()` method, which for `std::vector` takes a begin and end
iterator, instead of a begin iterator and a size, and replacing the
`append()` method with an equivalent `insert()`.

These changes have also been sent upstream [2].

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] https://gitlab.isc.org/isc-projects/kea/-/merge_requests/2410

PR:		280696
Approved by:	apevnev@me.com (maintainer)
MFH:		2024Q3