feat: action v2 signing (#8779)

# Which Problems Are Solved

The action v2 messages were didn't contain anything providing security
for the sent content.

# How the Problems Are Solved

Each Target now has a SigningKey, which can also be newly generated
through the API and returned at creation and through the Get-Endpoints.
There is now a HTTP header "Zitadel-Signature", which is generated with
the SigningKey and Payload, and also contains a timestamp to check with
a tolerance if the message took to long to sent.

# Additional Changes

The functionality to create and check the signature is provided in the
pkg/actions package, and can be reused in the SDK.

# Additional Context

Closes #7924

---------

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

cmd/start/start.go

@@ -196,6 +196,7 @@ func startZitadel(ctx context.Context, config *Config, masterKey string, server
 		keys.OTP,
 		keys.OIDC,
 		keys.SAML,
+		keys.Target,
 		config.InternalAuthZ.RolePermissionMappings,
 		sessionTokenVerifier,
 		func(q *query.Queries) domain.PermissionCheck {
@@ -245,6 +246,7 @@ func startZitadel(ctx context.Context, config *Config, masterKey string, server
 		keys.DomainVerification,
 		keys.OIDC,
 		keys.SAML,
+		keys.Target,
 		&http.Client{},
 		permissionCheck,
 		sessionTokenVerifier,