Replace shared_ptr in bignum with unique_ptr

[ruslan-ilesik]

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Custom destructor is required as type for openssl_bignum is incomplete at compile time and my
g++ (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
on wsl2 refuses to compile it with error

In file included from /usr/include/c++/13/memory:78,
from /home/ilesik/DPP/library/../include/dpp/nlohmann/json_fwd.hpp:14,
from /home/ilesik/DPP/library/../include/dpp/json_fwd.h:25,
from /home/ilesik/DPP/library/../include/dpp/snowflake.h:24,
from /home/ilesik/DPP/library/../include/dpp/dpp.h:36,
from /home/ilesik/DPP/src/unittest/test.h:27,
from /home/ilesik/DPP/src/unittest/test.cpp:22:
/usr/include/c++/13/bits/unique_ptr.h: In instantiation of ‘void std::default_delete<_Tp>::operator()(_Tp*) const [with _Tp = dpp::openssl_bignum]’:
/usr/include/c++/13/bits/unique_ptr.h:404:17: required from ‘std::unique_ptr<_Tp, _Dp>::~unique_ptr() [with _Tp = dpp::openssl_bignum; _Dp = std::default_deletedpp::openssl_bignum]’
/home/ilesik/DPP/library/../include/dpp/bignum.h:88:2: required from here
/usr/include/c++/13/bits/unique_ptr.h:97:23: error: invalid application of ‘sizeof’ to incomplete type ‘dpp::openssl_bignum’
97 | static_assert(sizeof(_Tp)>0,
| ^~~~~~~~~~~

  Given work around solves this issue.

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