Introduce oauth2scope registry with freeze lifecycle

Replace pkg/iam/scopeset with pkg/iam/oauth2scope.Registry, a shared
OAuth2 scope→action registry used by the authorizer, OAuth2 service,
and Connect API. Registration stays open until probod calls Freeze();
read paths (RegisteredScopes, Allows, ValidateScopes) panic before
that.

Drop the leaky APIScopes surface and AllowedAPIScopes on manual
access-token creation in favor of registry.ValidateScopes. Metadata,
protected-resource metadata, and CIMD scope lists are built from
RegisteredScopes() via helpers in pkg/iam/oauth2/scopes.go. Expose
oauth2ScopesSupported as an OAuth2Scope GraphQL scalar.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

pkg/iam/authorizer_oauth2_scope_test.go

@@ -24,7 +24,7 @@ import (
 	"go.probo.inc/probo/pkg/coredata"
 	"go.probo.inc/probo/pkg/gid"
 	"go.probo.inc/probo/pkg/iam/oauth2"
-	"go.probo.inc/probo/pkg/iam/scopeset"
+	"go.probo.inc/probo/pkg/iam/oauth2scope"
 )
 
 func TestAuthorizer_checkOAuth2Scope(t *testing.T) {
@@ -66,7 +66,7 @@ func TestAuthorizer_checkOAuth2Scope(t *testing.T) {
 	t.Run("allows when registered scopes authorize the action", func(t *testing.T) {
 		t.Parallel()
 
-		scopeSet := scopeset.New().Register(
+		scopeSet := oauth2scope.NewRegistry().Register(
 			map[coredata.OAuth2Scope][]string{
 				scopeV1OrgRead: {action},
 			},