Track util::Expected compatibility refactors for future std::expected migration #35332

Summary

In C++23 std::expected is introduced. Currently, we support C++20 and above because of this we use util::Expected.

I've opened a few PR's, making changes to use util::Expected now, and expect that there are more that I missed. I want to use this issue to consolidate everywhere we might need to refactor to use util::Expected instead.

PRs refactoring to use util::Expected

• #35201
• #35105
• #32958

Found these instances where util::Expected is alreayd used

grep -nri "util::Expected" ./src/
./src/index/txospenderindex.cpp:141:util::Expected<TxoSpender, std::string> TxoSpenderIndex::ReadTransaction(const CDiskTxPos& tx_pos) const
./src/index/txospenderindex.cpp:160:util::Expected<std::optional<TxoSpender>, std::string> TxoSpenderIndex::FindSpender(const COutPoint& txo) const
./src/index/txospenderindex.cpp:180:    return util::Expected<std::optional<TxoSpender>, std::string>(std::nullopt);
./src/index/txospenderindex.h:44:    util::Expected<TxoSpender, std::string> ReadTransaction(const CDiskTxPos& pos) const;
./src/index/txospenderindex.h:68:    util::Expected<std::optional<TxoSpender>, std::string> FindSpender(const COutPoint& txo) const;
./src/node/blockstorage.h:319:    using ReadRawBlockResult = util::Expected<std::vector<std::byte>, ReadRawError>;
./src/net_processing.cpp:534:    util::Expected<void, std::string> FetchBlock(NodeId peer_id, const CBlockIndex& block_index) override
./src/net_processing.cpp:1961:util::Expected<void, std::string> PeerManagerImpl::FetchBlock(NodeId peer_id, const CBlockIndex& block_index)
./src/net_processing.h:110:    virtual util::Expected<void, std::string> FetchBlock(NodeId peer_id, const CBlockIndex& block_index) = 0;
./src/util/threadpool.h:184:    [[nodiscard]] util::Expected<Future<F>, SubmitError> Submit(F&& fn) noexcept EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)
./src/util/threadpool.h:220:    [[nodiscard]] util::Expected<std::vector<RangeFuture<R>>, SubmitError> Submit(R&& fns) noexcept EXCLUSIVE_LOCKS_REQUIRED(!m_mutex)

Also, when Bitcoin Core upgrades to C++23, then we should ideally move to using std:expected instead