[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

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### 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/server.go

@@ -68,6 +68,11 @@ type BaseServer struct {
 
 	proxyAuthClose func()
 
+	// grpcExtensions holds additional gRPC services, interceptors, and shutdown
+	// hooks registered by external modules via RegisterGRPCExtension. Populated
+	// during boot (single-threaded), consumed by GRPCServer() and Stop().
+	grpcExtensions []GRPCExtension
+
 	listener    net.Listener
 	certManager *autocert.Manager
 	update      *version.Update
@@ -257,6 +262,7 @@ func (s *BaseServer) Stop() error {
 		s.proxyAuthClose()
 		s.proxyAuthClose = nil
 	}
+	runExtensionShutdownHooks(ctx, s.grpcExtensions)
 	_ = s.Store().Close(ctx)
 	_ = s.EventStore().Close(ctx)
 	if s.update != nil {