Allow AWS SES sourceArn to be set

[hardillb]

part of #4173

Description

Allows mail to be sent via AWS SES with a authorised identity

Related Issue(s)

#4173

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production

Labels

• Includes a DB migration? -> add the area:migration label

[hardillb]

Has a paired Helm PR FlowFuse/helm#438

Need to work out a way to test this before merging, but should be able to test with localfs and the right AWS credentials

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.61%. Comparing base (01934d5) to head (d6c04ea).

Files	Patch %	Lines
forge/postoffice/index.js	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4177      +/-   ##
==========================================
- Coverage   78.62%   78.61%   -0.02%     
==========================================
  Files         286      286              
  Lines       13097    13100       +3     
  Branches     2926     2928       +2     
==========================================
  Hits        10298    10298              
- Misses       2799     2802       +3     

Flag	Coverage Δ
backend	78.61% <0.00%> (-0.02%)	⬇️

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[hardillb]

Paired with FlowFuse/helm#438 for the helm config

[knolleary]

Both this and helm prs approved. Will leave merging at your discretion @hardillb