[management] Generic gRPC extension seam for external modules (#6894)

## Describe your changes

This adds an extension point to the management server for registering
additional gRPC services. We already have a generic integrations system
and dependency injection for server components. This closes the gap on
being able to also extend the gRPC API cleanly.

## Issue ticket number and link

N/A

## Stack

<!-- branch-stack -->

### Checklist
- [ ] Is it a bug fix
- [ ] Is a typo/documentation fix
- [x] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)
- [ ] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— **OR** I have discussed it with the NetBird team beforehand (link the
issue / Slack thread in the description). See
[CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first).

> By submitting this pull request, you confirm that you have read and
agree to the terms of the [Contributor License
Agreement](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTOR_LICENSE_AGREEMENT.md).

## Documentation
Select exactly one:

- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change (explain why)

No docs needed. This is strictly a small internal plumbing enhancement /
refactor.

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added a gRPC extension mechanism to contribute additional services and
automatically chain extra unary and stream interceptors.
  * Extension shutdown hooks now run as part of server stop.
* Added exported proxy token generation via `GenerateProxyToken()` for
external integrations.
* **Tests**
* Added coverage for extension interceptor/service wiring, extension
shutdown execution, and proxy token generation validation (including
hash consistency and prefix).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

management/internals/server/boot.go

@@ -24,13 +24,13 @@ import (
 
 	"github.com/netbirdio/netbird/encryption"
 	"github.com/netbirdio/netbird/formatter/hook"
+	"github.com/netbirdio/netbird/management/internals/modules/agentnetwork"
 	"github.com/netbirdio/netbird/management/internals/modules/reverseproxy/accesslogs"
 	accesslogsmanager "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/accesslogs/manager"
 	rpservice "github.com/netbirdio/netbird/management/internals/modules/reverseproxy/service"
 	nbgrpc "github.com/netbirdio/netbird/management/internals/shared/grpc"
 	"github.com/netbirdio/netbird/management/server/activity"
 	activitystore "github.com/netbirdio/netbird/management/server/activity/store"
-	"github.com/netbirdio/netbird/management/internals/modules/agentnetwork"
 	nbcache "github.com/netbirdio/netbird/management/server/cache"
 	nbContext "github.com/netbirdio/netbird/management/server/context"
 	nbhttp "github.com/netbirdio/netbird/management/server/http"
@@ -184,6 +184,10 @@ func (s *BaseServer) GRPCServer() *grpc.Server {
 			grpc.ChainStreamInterceptor(realip.StreamServerInterceptorOpts(realipOpts...), streamInterceptor, proxyStream),
 		}
 
+		// Append interceptors contributed by registered gRPC extensions. These
+		// run after the built-in chain (ChainUnaryInterceptor is additive).
+		gRPCOpts = appendExtensionInterceptors(gRPCOpts, s.grpcExtensions)
+
 		if s.Config.HttpConfig.LetsEncryptDomain != "" {
 			certManager, err := encryption.CreateCertManager(s.Config.Datadir, s.Config.HttpConfig.LetsEncryptDomain)
 			if err != nil {
@@ -215,6 +219,9 @@ func (s *BaseServer) GRPCServer() *grpc.Server {
 		mgmtProto.RegisterProxyServiceServer(gRPCAPIHandler, s.ReverseProxyGRPCServer())
 		log.Info("ProxyService registered on gRPC server")
 
+		// Register services contributed by external modules via the extension seam.
+		registerExtensions(gRPCAPIHandler, s.grpcExtensions)
+
 		return gRPCAPIHandler
 	})
 }