Email action - user and password should be optional

[peterschretlen]

Describe the feature:

The email action requires user and password in its secrets schema, even though an SMTP service is not required to have authentication.

Describe a specific use case for the feature:

Connecting to an internal SMTP service that does not use authentication. Currently this will give an
error like:

{
  "status": "error",
  "actionId": "...",
  "message": "error sending email",
  "serviceMessage": "Invalid login: 503 5.5.1 Error: authentication not enabled"
}

See also #50646

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[pmuellr]

So I think we want to make the secrets properties optional, using the "nullable" schema function to have them saveable as a null:

kibana/x-pack/plugins/actions/server/builtin_action_types/email.ts

Lines 77 to 80 in cf9b64e

	const SecretsSchema = schema.object({
	user: schema.string(),
	password: schema.string(),
	});

And then not set them in the transport object we send to nodemailer, if they are null:

kibana/x-pack/plugins/actions/server/builtin_action_types/email.ts

Lines 147 to 150 in cf9b64e

	const transport: any = {
	user: secrets.user,
	password: secrets.password,
	};

I wonder if we'll need some additional nodemailer options, for some reason. We'll see.