🎯 Summary

PIMS-1370

• This implements the CHES service for sending email notifications. Does not include templates.
• I have added an additional endpoint for sending emails that was not in the original spec, previously the send service was only called internally as part of the notification system, which is currently unimplemented. I figure I'll leave this in until it's deemed unnecessary, since I don't know whether the notification system will be kept/discarded/reworked etc.
• You'll notice that a lot of environment variables are expected. Previously this configuration was handled by the .NET appsettings json configuration, so this is my equivalent, though I'm open to suggestions on another way to do it.

🔰 Checklist

• I have read and agree with the following checklist and am following the guidelines in our Code of Conduct document.

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation where required.
• I have tested my changes to the best of my ability.
• My changes generate no new warnings.