batsignal: add module

[loicreynier]

Description

Adds batsignal module.

Checklist

• [x] Change is backwards compatible.

• [x] Code formatted with ./format.

• [x] Code tested through nix-shell --pure tests -A run.all.

• [x] Test cases updated/added. See example.

• [x] Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • [x] Added myself as module maintainer. See example.

  • [x] Added myself and the module files to .github/CODEOWNERS.

[loicreynier]

I wanted to add the dangerLevelCommand option to the test, but since it generates an entry in the store for the generated shell script I don't know how to deal with it:

[Service]
ExecStart=@batsignal@/bin/batsignal -w 15 -c 5 -d 2 -f 5 -D /nix/store/fv4mq8q355016vraa178bndbqa75w4rx-batsignal-dangercmd -n 'Internal' -m 120 -a 'Batsignal daemon' -I battery