Add missing validations for the REQUIRED configurations.

**Description:**
In the Configurations catalog added with https://github.com/asgardio/asgardio-tomcat-oidc-agent/pull/1/commits/0ec2633adb762bf76cee7252e206912c38cb3e59, some properties are marked as REQUIRED in the app.properties file.
In the config provider or in the DefaultOIDCManager constructor, we need to validate these properties that are marked as REQUIRED.

As of now, validations for the following properties are implemented.

- Consumer Key / Client ID
- Consumer Secret / Client Secret
- Callback URI / Redirect URI
- Scope

Missing validations for the other REQUIRED parameters must also be added there.

Start by reviewing the REQUIRED entries in app.properties and the existing validation for Consumer Key, Consumer Secret, Callback URI, and Scope. Then inspect the config provider and DefaultOIDCManager constructor to identify the remaining REQUIRED parameters; done means each is validated consistently with the existing checks.

Written by the indexing model from the issue text.