feat: http provider signing key addition (#10641)

# Which Problems Are Solved

HTTP Request to HTTP providers for Email or SMS are not signed.

# How the Problems Are Solved

Add a Signing Key to the HTTP Provider resources, which is then used to
generate a header to sign the payload.

# Additional Changes

Additional tests for query side of the SMTP provider.

# Additional Context

Closes #10067

---------

Co-authored-by: Marco A. <marco@zitadel.com>
(cherry picked from commit 8909b9a2a6)

internal/notification/channels/webhook/config.go

@@ -6,9 +6,10 @@ import (
 )
 
 type Config struct {
-	CallURL string
-	Method  string
-	Headers http.Header
+	CallURL    string
+	Method     string
+	Headers    http.Header
+	SigningKey string
 }
 
 func (w *Config) Validate() error {