Merge bitcoin#27218: util: Work around ParseHex gcc cross compiler bug

fa8481b util: Work around ParseHex gcc cross compiler bug (MarcoFalke)

Pull request description:

  I fail to see how an explicit `ParseHex` template instantiation fails to also instantiate `TryParseHex`.

  Nonetheless, to work around a compiler bug, change the explicit instantiation from `ParseHex` to `TryParseHex`. (`ParseHex` is inline anyway and will be instantiated by the compiler either way).

  Fixes bitcoin#25227 (comment) :

  ```
    CXXLD    bitcoind
  /usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-net_processing.o): in function `(anonymous namespace)::PeerManagerImpl::ProcessMessage(CNode&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, CDataStream&, std::chrono::duration<long, std::ratio<1l, 1000000l> >, std::atomic<bool> const&)':
  net_processing.cpp:(.text+0x29660): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
  /usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-rest.o): in function `rest_getutxos(std::any const&, HTTPRequest*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
  rest.cpp:(.text+0x83b4): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
  /usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_node.a(libbitcoin_node_a-torcontrol.o): in function `std::vector<unsigned char, std::allocator<unsigned char> > ParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)':
  torcontrol.cpp:(.text._Z8ParseHexIhESt6vectorIT_SaIS1_EESt17basic_string_viewIcSt11char_traitsIcEE[_Z8ParseHexIhESt6vectorIT_SaIS1_EESt17basic_string_viewIcSt11char_traitsIcEE]+0x2c): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
  /usr/lib/gcc-cross/powerpc64le-linux-gnu/11/../../../../powerpc64le-linux-gnu/bin/ld: libbitcoin_common.a(libbitcoin_common_a-external_signer.o): in function `ExternalSigner::SignTransaction(PartiallySignedTransaction&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&)':
  external_signer.cpp:(.text+0x8d84): undefined reference to `std::optional<std::vector<unsigned char, std::allocator<unsigned char> > > TryParseHex<unsigned char>(std::basic_string_view<char, std::char_traits<char> >)'
  collect2: error: ld returned 1 exit status

ACKs for top commit:
  gruve-p:
    ACK bitcoin@fa8481b
  hebasto:
    ACK fa8481b, tested on Ubuntu 22.04, gcc 11.3 for the `riscv64-linux-gnu` host.,

Tree-SHA512: 53efa424e7e18d85a2c9ac2267b9370ae3594d9be73da5135a3a79bf07ab50fcc5357cbde09dc0b2a9eb78d78ec37beae0c9f876333b568e678b9d0067bc9e4e

src/util/strencodings.cpp

@@ -95,8 +95,8 @@ std::optional<std::vector<Byte>> TryParseHex(std::string_view str)
     }
     return vch;
 }
-template std::vector<std::byte> ParseHex(std::string_view);
-template std::vector<uint8_t> ParseHex(std::string_view);
+template std::optional<std::vector<std::byte>> TryParseHex(std::string_view);
+template std::optional<std::vector<uint8_t>> TryParseHex(std::string_view);
 
 bool SplitHostPort(std::string_view in, uint16_t& portOut, std::string& hostOut)
 {