Validate that Okta audience config is an absolute URL

Add a check in the Okta auth provider initialization that ensures
the provided audience is an absolute URL with an http(s) scheme,
and throws a descriptive error if not.

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/okta-audience-url-validation.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-auth-backend-module-okta-provider': patch
+---
+
+Added a validation check that rejects `audience` configuration values that are not absolute URLs (i.e. missing `https://` or `http://` prefix).