databases/mysql84-server: fix build with clang and 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 databases/mysql84-server to fail to compile with clang 19
and libc++ 19, resulting in errors similar to:

/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,

      |                                          ^
/wrkdirs/usr/ports/databases/mysql84-client/work/mysql-8.4.2/sql/rpl_log_encryption.h:821:14:

note: in instantiation of template class 'std::basic_string<unsigned
char>' requested here

  821 |   Key_string m_encrypted_password;
      |              ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared

here

23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
   |                             ^

Key_string 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 only a few small adjustments in other places: replacing
the length() method with the equivalent size() method, and 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.

Another issue is that clang 19 now implements CWG 96 [2], which requires
a template argument list after a 'template' keyword, resulting in errors
similar to:

/wrkdirs/usr/ports/databases/mysql84-client/work/mysql-8.4.2/libs/mysql/serialization/archive.h:84:43:

error: a template argument list is expected after a name prefixed by the
template keyword [-Wmissing-template-arg-list-after-template-kw]

84 |     return Archive_derived_type::template

get_size(std::forward<Type>(arg));

      |                                           ^
/wrkdirs/usr/ports/databases/mysql84-client/work/mysql-8.4.2/libs/mysql/serialization/archive.h:91:43:

error: a template argument list is expected after a name prefixed by the
template keyword [-Wmissing-template-arg-list-after-template-kw]

91 |     return Archive_derived_type::template

get_size_written();

|                                           ^

This requires a number of adjustments to various template invocations.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] https://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#96

PR: 280695
Sponsored by: Netzkommune GmbH