feat: add stdout and filesystem notification channels (#2925)

* feat: add filesystem and stdout notification channels

* configure through env vars

* compile

* feat: add compact option for debug notification channels

* fix channel mock generation

* avoid sensitive information in error message

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

* add review improvements

Co-authored-by: Livio Amstutz <livio.a@gmail.com>

internal/notification/messages/email.go

@@ -0,0 +1,51 @@
+package messages
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+
+	"github.com/caos/zitadel/internal/notification/channels"
+)
+
+var (
+	isHTMLRgx = regexp.MustCompile(`.*<html.*>.*`)
+	lineBreak = "\r\n"
+)
+
+var _ channels.Message = (*Email)(nil)
+
+type Email struct {
+	Recipients  []string
+	BCC         []string
+	CC          []string
+	SenderEmail string
+	Subject     string
+	Content     string
+}
+
+func (msg *Email) GetContent() string {
+	headers := make(map[string]string)
+	headers["From"] = msg.SenderEmail
+	headers["To"] = strings.Join(msg.Recipients, ", ")
+	headers["Cc"] = strings.Join(msg.CC, ", ")
+
+	message := ""
+	for k, v := range headers {
+		message += fmt.Sprintf("%s: %s"+lineBreak, k, v)
+	}
+
+	//default mime-type is html
+	mime := "MIME-version: 1.0;" + lineBreak + "Content-Type: text/html; charset=\"UTF-8\";" + lineBreak + lineBreak
+	if !isHTML(msg.Content) {
+		mime = "MIME-version: 1.0;" + lineBreak + "Content-Type: text/plain; charset=\"UTF-8\";" + lineBreak + lineBreak
+	}
+	subject := "Subject: " + msg.Subject + lineBreak
+	message += subject + mime + lineBreak + msg.Content
+
+	return message
+}
+
+func isHTML(input string) bool {
+	return isHTMLRgx.MatchString(input)
+}