break identity client into an interface

The interface has changed a little instead of allowing the client to
parse out the authorization header, it takes the request object as is
to extract the identity from it how the implementation decides.

IdentityClient#authenticate is now deprecated, in favor of
IdentityApi#getIdentity.

I am leaving the IdentityClient in place deprecated so that plugins
that use this can migrate away from it.

Signed-off-by: Brian Fletcher <brian@roadie.io>

docs/permissions/plugin-authors/01-setup.md

@@ -46,7 +46,7 @@ The source code is available here:
     Create a new `packages/backend/src/plugins/todolist.ts` with the following content:
 
     ```javascript
-    import { IdentityClient } from '@backstage/plugin-auth-node';
+    import { DefaultIdentityClient } from '@backstage/plugin-auth-node';
     import { createRouter } from '@internal/plugin-todo-list-backend';
     import { Router } from 'express';
     import { PluginEnvironment } from '../types';
@@ -57,7 +57,7 @@ The source code is available here:
     }: PluginEnvironment): Promise<Router> {
       return await createRouter({
         logger,
-        identity: IdentityClient.create({
+        identity: DefaultIdentityClient.create({
           discovery,
           issuer: await discovery.getExternalBaseUrl('auth'),
         }),