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>

.changeset/thin-cows-watch.md

@@ -0,0 +1,7 @@
+---
+'@internal/plugin-todo-list-backend': patch
+'@backstage/plugin-permission-backend': patch
+'@backstage/plugin-scaffolder-backend': patch
+---
+
+Uptake the IdentityApi change to use getIdentiy instead of authenticate for retrieving the logged in users identity.