Use a custom type for our static CLI client (smaller change).

Before this change, we used the `fosite.DefaultOpenIDConnectClient{}` struct, which implements the  `fosite.Client` and `fosite.OpenIDConnectClient` interfaces. For a future change, we also need to implement some additional optional interfaces, so we can no longer use the provided default types. Instead, we now use a custom `clientregistry.Client{}` struct, which implements all the requisite interfaces and can be extended to handle the new functionality (in a future change).

There is also a new `clientregistry.StaticRegistry{}` struct, which implements the `fosite.ClientManager` and looks up our single static client. We could potentially extend this in the future with a registry backed by Kubernetes API, for example.

This should be 100% refactor, with no user-observable change.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

internal/fositestorage/authorizationcode/authorizationcode.go

@@ -1,4 +1,4 @@
-// Copyright 2020 the Pinniped contributors. All Rights Reserved.
+// Copyright 2020-2021 the Pinniped contributors. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
 package authorizationcode
@@ -18,6 +18,7 @@ import (
 	"go.pinniped.dev/internal/constable"
 	"go.pinniped.dev/internal/crud"
 	"go.pinniped.dev/internal/fositestorage"
+	"go.pinniped.dev/internal/oidc/clientregistry"
 )
 
 const (
@@ -137,7 +138,7 @@ func (a *authorizeCodeStorage) getSession(ctx context.Context, signature string)
 func NewValidEmptyAuthorizeCodeSession() *AuthorizeCodeSession {
 	return &AuthorizeCodeSession{
 		Request: &fosite.Request{
-			Client:  &fosite.DefaultOpenIDConnectClient{},
+			Client:  &clientregistry.Client{},
 			Session: &openid.DefaultSession{},
 		},
 	}