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/server/api/connect/v1/graphql/base.graphql

@@ -16,6 +16,7 @@ scalar CursorKey
 scalar Datetime
 scalar Upload
 scalar EmailAddr
+scalar OAuth2Scope
 
 interface Node {
   id: ID!
@@ -33,7 +34,7 @@ type Query {
   signUpEnabled: Boolean!
     @goField(forceResolver: true)
     @authentication(required: OPTIONAL)
-  oauth2ScopesSupported: [String!]!
+  oauth2ScopesSupported: [OAuth2Scope!]!
     @goField(forceResolver: true)
     @authentication(required: OPTIONAL)
 }