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>

plugins/auth-node/package.json

@@ -26,6 +26,7 @@
     "@backstage/backend-common": "^0.14.1-next.1",
     "@backstage/config": "^1.0.1",
     "@backstage/errors": "^1.1.0-next.0",
+    "express": "^4.18.1",
     "jose": "^4.6.0",
     "node-fetch": "^2.6.7",
     "winston": "^3.2.1"