Move towards compile-time type safety for arg registration

[ajtowns]

Introduce ArgsRegister for defining arg parsing in a single location.

Idea is that we have two types, the FieldType (eg RegTestOptions::version_bits_parameters has type std::unordered_map<Consensus::DeploymentPos, VersionBitsParameter>) and the ArgType for which we specialise std::optional<ArgType> ArgsManager::Get<ArgType>(std::string& argname in common/args.cpp. Then, any argument that we register via ArgsRegister either has FieldType = ArgType or FieldType = std::optional<ArgType> or has an explicit conversion function.

In future we can then also specialise ArgsManager::AddTypedArg<FieldType> to give earlier errors (eg by automatically adding ALLOW_LIST etc).

[ajtowns]

Maybe this should have a namespace, eg:

template<typename T>
class ArgsRegisterImpl<T>;

template<>
class ArgsRegisterImpl<CChainParams::RegTestOptions>
{
public:
    ...
}

Then you can just say ArgsRegister<CChainParams::RegTestOptions> later.

[ajtowns]

These could be outside the class, just regular static module level functions.

[ajtowns]

Maybe C::Register(op, C::Arg(..)) would be better names?

[ajtowns]

Maybe ArgsRegister<..>::ApplyArgs would be better naming? (cf node/mempool_args.h)

[ajtowns]

-fooc

[ajtowns]

(Using ArgsRegisterImpl naming would make this be template<typename STRUCT> class ArgsRegister { public: using REG = ArgsRegisterImpl<T>; instead)