feat: backend support for creating and storing service accounts (#22698)

Add is_service_account column to users table with CHECK constraints
enforcing login_type='none' and empty email for service accounts.
Update user creation API to validate service account constraints.

Related to:
https://linear.app/codercom/issue/PLAT-27/feat-backend-support-for-creating-and-storing-service-accounts

coderd/apidoc/swagger.json

@@ -12856,7 +12856,7 @@
 		},
 		"codersdk.CreateUserRequestWithOrgs": {
 			"type": "object",
-			"required": ["email", "username"],
+			"required": ["username"],
 			"properties": {
 				"email": {
 					"type": "string",
@@ -12884,6 +12884,10 @@
 				"password": {
 					"type": "string"
 				},
+				"service_account": {
+					"description": "Service accounts are admin-managed accounts that cannot login.",
+					"type": "boolean"
+				},
 				"user_status": {
 					"description": "UserStatus defaults to UserStatusDormant.",
 					"allOf": [