feat(actionsv2): Propagate request headers in actions v2 (#10632)

# Which Problems Are Solved

This PR adds functionality to propagate request headers in actions v2.

# How the Problems Are Solved
The new functionality is added to the`ExecutionHandler` interceptors,
where the incoming request headers (from a list of allowed headers to be
forwarded) are set in the payload of the request before calling the
target.

# Additional Changes
This PR also contains minor fixes to the Actions V2 example docs.

# Additional Context
- Closes #9941

---------

Co-authored-by: Marco A. <marco@zitadel.com>
(cherry picked from commit 51e12e224d)

internal/api/http/header.go

@@ -26,6 +26,7 @@ const (
 	ForwardedHost          = "x-forwarded-host"
 	ForwardedProto         = "x-forwarded-proto"
 	Forwarded              = "forwarded"
+	Host                   = "host"
 	ZitadelForwarded       = "x-zitadel-forwarded"
 	XUserAgent             = "x-user-agent"
 	XGrpcWeb               = "x-grpc-web"