[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/server/types/proxy_access_token.go

@@ -68,7 +68,7 @@ type ProxyAccessTokenGenerated struct {
 // CreateNewProxyAccessToken generates a new proxy access token.
 // Returns the token with hashed value stored and plain token for one-time display.
 func CreateNewProxyAccessToken(name string, expiresIn time.Duration, accountID *string, createdBy string) (*ProxyAccessTokenGenerated, error) {
-	hashedToken, plainToken, err := generateProxyToken()
+	hashedToken, plainToken, err := GenerateProxyToken()
 	if err != nil {
 		return nil, err
 	}
@@ -94,7 +94,10 @@ func CreateNewProxyAccessToken(name string, expiresIn time.Duration, accountID *
 	}, nil
 }
 
-func generateProxyToken() (HashedProxyToken, PlainProxyToken, error) {
+// GenerateProxyToken generates a new random proxy token, returning its SHA-256
+// hash (for storage) and the one-time plaintext. Exported so external modules
+// can mint tokens in the canonical proxy-token format.
+func GenerateProxyToken() (HashedProxyToken, PlainProxyToken, error) {
 	secret, err := b.Random(ProxyTokenSecretLength)
 	if err != nil {
 		return "", "", err

management/server/types/proxy_access_token_test.go

@@ -1,6 +1,7 @@
 package types
 
 import (
+	"strings"
 	"testing"
 	"time"
 
@@ -123,6 +124,22 @@ func TestCreateNewProxyAccessToken(t *testing.T) {
 	})
 }
 
+func TestGenerateProxyToken(t *testing.T) {
+	hashed, plain, err := GenerateProxyToken()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := plain.Validate(); err != nil {
+		t.Errorf("generated token failed Validate(): %v", err)
+	}
+	if plain.Hash() != hashed {
+		t.Error("returned hashed token does not match Hash(plain)")
+	}
+	if !strings.HasPrefix(string(plain), ProxyTokenPrefix) {
+		t.Errorf("token %q missing prefix %q", plain, ProxyTokenPrefix)
+	}
+}
+
 func TestProxyAccessToken_IsExpired(t *testing.T) {
 	past := time.Now().Add(-1 * time.Hour)
 	future := time.Now().Add(1 * time.Hour)