params: clarify consensus engine config Strings

[roysc]

• Defines the String() methods on a value receiever so that nil is shown differently.
• Adds the parameters to the representation for CliqueConfig.

Currently these config objects will display the same whether they're nil or present. For example, this chain config:

{
  "chainId": 1212,
  "homesteadBlock": 0,
  "eip150Block": 0,
  "eip155Block": 0,
  "eip158Block": 0,
  "byzantiumBlock": 0,
  "constantinopleBlock": 0,
  "petersburgBlock": 0,
  "istanbulBlock": 0,
  "berlinBlock": 0,
  "londonBlock": 0,
  "mergeNetsplitBlock": 0,
  "terminalTotalDifficulty": 0,
  "shanghaiTime": 0,
  "cancunTime": 0
}

when printed with %+v, shows as (note Ethash:ethash Clique:clique at the end):

&{ChainID:+1212 HomesteadBlock:+0 DAOForkBlock:<nil> DAOForkSupport:false EIP150Block:+0 EIP155Block:+0 EIP158Block:+0 ByzantiumBlock:+0 ConstantinopleBlock:+0 PetersburgBlock:+0 IstanbulBlock:+0 MuirGlacierBlock:<nil> BerlinBlock:+0 LondonBlock:+0 ArrowGlacierBlock:<nil> GrayGlacierBlock:<nil> MergeNetsplitBlock:+0 ShanghaiTime:0x14000312da0 CancunTime:0x14000312da8 PragueTime:<nil> VerkleTime:<nil> TerminalTotalDifficulty:+0 Ethash:ethash Clique:clique}

With this change, we see Ethash:<nil> Clique:<nil>.

If we add a clique key to the config:

{
  "chainId": 1212,
  "homesteadBlock": 0,
  "eip150Block": 0,
  "eip155Block": 0,
  "eip158Block": 0,
  "byzantiumBlock": 0,
  "constantinopleBlock": 0,
  "petersburgBlock": 0,
  "istanbulBlock": 0,
  "berlinBlock": 0,
  "londonBlock": 0,
  "mergeNetsplitBlock": 0,
  "terminalTotalDifficulty": 0,
  "shanghaiTime": 0,
  "cancunTime": 0,
  "clique": {
    "period": 5,
    "epoch": 30000
  }
}

and print it, it currently shows identically to the first one (Ethash:ethash Clique:clique).

With this change we see Ethash:<nil> Clique:clique(period: 5, epoch: 30000).
This makes it more clear which engines are and are not active.

This is revised from #29635.

[holiman]

That was a pretty neat way to solve it, actually!