feat: exchange gRPC server implementation to connectRPC (#10145)

# Which Problems Are Solved

The current maintained gRPC server in combination with a REST (grpc)
gateway is getting harder and harder to maintain. Additionally, there
have been and still are issues with supporting / displaying `oneOf`s
correctly.
We therefore decided to exchange the server implementation to
connectRPC, which apart from supporting connect as protocol, also also
"standard" gRCP clients as well as HTTP/1.1 / rest like clients, e.g.
curl directly call the server without any additional gateway.

# How the Problems Are Solved

- All v2 services are moved to connectRPC implementation. (v1 services
are still served as pure grpc servers)
- All gRPC server interceptors were migrated / copied to a corresponding
connectRPC interceptor.
- API.ListGrpcServices and API. ListGrpcMethods were changed to include
the connect services and endpoints.
- gRPC server reflection was changed to a `StaticReflector` using the
`ListGrpcServices` list.
- The `grpc.Server` interfaces was split into different combinations to
be able to handle the different cases (grpc server and prefixed gateway,
connect server with grpc gateway, connect server only, ...)
- Docs of services serving connectRPC only with no additional gateway
(instance, webkey, project, app, org v2 beta) are changed to expose that
- since the plugin is not yet available on buf, we download it using
`postinstall` hook of the docs

# Additional Changes

- WebKey service is added as v2 service (in addition to the current
v2beta)

# Additional Context

closes #9483

---------

Co-authored-by: Elio Bischof <elio@zitadel.com>

internal/api/grpc/saml/v2/server.go

@@ -1,7 +1,10 @@
 package saml
 
 import (
-	"google.golang.org/grpc"
+	"net/http"
+
+	"connectrpc.com/connect"
+	"google.golang.org/protobuf/reflect/protoreflect"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
 	"github.com/zitadel/zitadel/internal/api/grpc/server"
@@ -9,9 +12,10 @@ import (
 	"github.com/zitadel/zitadel/internal/command"
 	"github.com/zitadel/zitadel/internal/query"
 	saml_pb "github.com/zitadel/zitadel/pkg/grpc/saml/v2"
+	"github.com/zitadel/zitadel/pkg/grpc/saml/v2/samlconnect"
 )
 
-var _ saml_pb.SAMLServiceServer = (*Server)(nil)
+var _ samlconnect.SAMLServiceHandler = (*Server)(nil)
 
 type Server struct {
 	saml_pb.UnimplementedSAMLServiceServer
@@ -38,8 +42,12 @@ func CreateServer(
 	}
 }
 
-func (s *Server) RegisterServer(grpcServer *grpc.Server) {
-	saml_pb.RegisterSAMLServiceServer(grpcServer, s)
+func (s *Server) RegisterConnectServer(interceptors ...connect.Interceptor) (string, http.Handler) {
+	return samlconnect.NewSAMLServiceHandler(s, connect.WithInterceptors(interceptors...))
+}
+
+func (s *Server) FileDescriptor() protoreflect.FileDescriptor {
+	return saml_pb.File_zitadel_saml_v2_saml_service_proto
 }
 
 func (s *Server) AppName() string {