Add --argjson similiar to --argstr to allow passing JSON into nix from the command-line.

[tomprince]

Is your feature request related to a problem? Please describe.
When using nix programatically, it is useful to pass in structured data to a nix expression.

Describe the solution you'd like
I'd like to be able to pass --argjson args '{"some": "args"}' and have the nix expression receive the nix value {some = "args";}, rather than needing to use --argstr and then builtins.fromJSON on the result. The would mean the expression being called doesn't need to make allowances for being called from outside nix, and can just take a structured value.

Describe alternatives you've considered
It is currently possible to do this via something like

# example.nix
{ argsJSON ? null, args ? builtins.fromJSON argsJSON }: /* stuff using args */

and running it with nix build -f example.nix --argstr '{"some": "args"}.

Additional context

For example, in https://github.com/DBCDK/morph/pull/186/files#diff-cf26037a329af2f212a4b7c055c4466aaa4d04fae33fc0a711a6353291025e57R111 I need some extra logic to deal with potentially being passed a JSON argument. If instead there was --argjson, I could just pass --argjson names '["node1", "node2"] and remove the logic in nix around argsJSON. I currently have code like the following

{
  names ? null,
  argsJSON ? null,
}:
  assert argsJSON != null -> names == null;
  let
    names' = if argsFile != null then
        (builtins.fromJSON argsJSON).names
      else
        names;
  in ...

but with this option, I could simply have

{ names }: ...`