feat: support plaintext SMTP relays (WI-1106) #1139

Job	Run time
Build (ws-carrier/amd64)	1m 56s
Build (ws-carrier/arm64)	1m 3s
Build (windshift-runner/arm64)	1m 49s
Build (windshift/arm64)	4m 3s
Build (windshift-runner/amd64)	2m 39s
Build (windshift/amd64)	6m 47s
Merge Manifests (ws-carrier)	34s
Merge Manifests (windshift)	1m 12s
Merge Manifests (windshift-runner)	35s
	20m 38s

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