Add a mailing function

[bryce-turner]

Slurm supports sending mail and we can add that as an sbatch_arg, but as we work towards more dynamic jetstream functionality, we may want to send mail without requiring a job submission.

For example, if we use an exec directive to parse an existing file within a project, we may want to {{ mail(user, 'insert message') }}

Workarounds can be made for this, such as parsing the file within the task and mailing via bash, including some slurm job signalling, but a backend agnostic function might be useful.

This could also relate to additional mailing functionality within jetstream_centro - we could create a mail template at runtime and then jetstream_centro uses that to email users related to the run submission.