feat universal: add insert method to SmallString

[fdr400]

fixes:

-c /Users/runner/work/userver/userver/universal/src/logging/impl/formatters/tskv.cpp
In file included from /Users/runner/work/userver/userver/universal/src/logging/impl/formatters/tskv.cpp:5:
In file included from /opt/homebrew/include/fmt/chrono.h:23:
In file included from /opt/homebrew/include/fmt/format.h:41:
/opt/homebrew/include/fmt/base.h:2027:5: error: no member named 'insert' in 'userver::utils::SmallString<4096>'
  c.insert(c.end(), begin, end);
  ~ ^
/opt/homebrew/include/fmt/format.h:535:10: note: in instantiation of function template specialization 'fmt::detail::copy<char, const char *, std::back_insert_iterator<userver::utils::SmallString<4096>>, 0>' requested here
  return copy<OutChar>(begin, end, out);

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[botanegg]