Support forkid in ENR

[gumb0]

Addresses #5889

TODO

• construct forkHash in BlockChain::forkHash()
  • BlockChain::m_params has all the known fork block numbers. Fork block number = c_infiniteBlockNumber means it's disabled; fork block number = 0 means it's activated at genesis.
  • libcryptopp can be used to calculate CRC32 https://github.com/weidai11/cryptopp/blob/5d688500414f138491f2ae0d298f9378656fde3c/crc.h#L25
    try to just #include <cryptopp/crc.h> but better to create a wrapper for it in our libdevcrypto
• return forkNext from BlockChain::nextForkBlockNumber()
• add eth field to ENR - easiest would be to add it to IdentitySchemeV4::updateENR and IdentitySchemeV4::createKeyValuePairs
• pass forkHash and forkNext from NodeTable to IdentitySchemeV4::updateENR

cc @fjl