[Upstream PR#313] Signal middleware

[CybotTM]

Original PR

mcuadros#313

Summary

Add Signal messenger notification middleware.

Decision Needed

• Port from upstream PR
• Won't implement (reason: ___)

[CybotTM]

Addressed in #395 (merged as 5b60847). Signal can be supported via a custom webhook preset using Signal CLI REST API or similar bridges.

Example custom preset:

name: signal
url_scheme: "http://signal-cli:8080/v2/send"
method: POST
headers:
  Content-Type: application/json
body: |
  {"message": "Job {{.Job.Name}}: {{.Execution.Status}}", "number": "{id}", "recipients": ["{secret}"]}

Use with: preset = /path/to/signal.yaml