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/server/connect_middleware/validation_interceptor.go

@@ -0,0 +1,36 @@
+package connect_middleware
+
+import (
+	"context"
+
+	"connectrpc.com/connect"
+	// import to make sure go.mod does not lose it
+	// because dependency is only needed for generated code
+	_ "github.com/envoyproxy/protoc-gen-validate/validate"
+)
+
+func ValidationHandler() connect.UnaryInterceptorFunc {
+	return func(handler connect.UnaryFunc) connect.UnaryFunc {
+		return func(ctx context.Context, req connect.AnyRequest) (connect.AnyResponse, error) {
+			return validate(ctx, req, handler)
+		}
+	}
+}
+
+// validator interface needed for github.com/envoyproxy/protoc-gen-validate
+// (it does not expose an interface itself)
+type validator interface {
+	Validate() error
+}
+
+func validate(ctx context.Context, req connect.AnyRequest, handler connect.UnaryFunc) (connect.AnyResponse, error) {
+	validate, ok := req.Any().(validator)
+	if !ok {
+		return handler(ctx, req)
+	}
+	err := validate.Validate()
+	if err != nil {
+		return nil, connect.NewError(connect.CodeInvalidArgument, err)
+	}
+	return handler(ctx, req)
+}