feat: big number implementation, for future use in permissions bitfields

[braindigitalis]

Officially, the Discord documentation says that permission values can be any arbitrary number of digits. At time of writing there are only 50 bits of permissions, but this is set to grow larger and potentially past 64 bits. They will continue to send this data as a huge single integer at that point, because this is obviously sensible. /s

dpp::bignumber uses OpenSSL BN_* under the hood, as we include openssl anyway for HTTPS.

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