Add mongodb service

[obinmatt]

Closes #1062

This is my first attempt at anything launchd related so bear with me. I used the postgres implementation as an example for this one combined with nixos module.

Details:

• I went with a launchd.agent so the user option does nothing.
• For the auth, the nixos module uses systemd-run to run the command, I don't think there is an equivalent for launchd so I omitted that step.
• For the dbpath I pass it as an option to the exec command instead of adding it into the mongodb.conf file because the file cannot handle ~ in paths.
• I formatted the files using nixfmt.

If I need to make any changes please let me know!