nix store ping: add --json flag

[yorickvP]

Motivation

Requested in #7515 (comment)

Context

I also tried to add a Finally instance to also print the json in case of error, but the store connection happens (and fails) before the run() method is even entered, so this doesn't work. nix show-config --json does work in that case.

Checklist for maintainers

Maintainers: tick if completed or explain if not relevant

• agreed on idea
• agreed on implementation strategy
• tests, as appropriate
  • functional tests - tests/**.sh
  • unit tests - src/*/tests
  • integration tests - tests/nixos/*
• documentation in the manual
• code and comments are self-explanatory
• commit message explains why the change was made
• new feature or bug fix: updated release notes

[thufschmitt]

Looks good overall 👍

Can you just add a quick test for it? (there's already a store-ping test, you can just extend it)

[roberth]

cc @MatthewCroughan

Comments addressed.