Refactoring token expiry check

[ash1425]

We had introduced additional abstract class to add token expiry awareness in last release. This made it 3 levels of abstract classes.

After looking into it closely, I think probably it's a good idea to make token expiry aware a default behaviour - as this is useful in all the cases except plain token based secret.

About duplication of token expiry checking logic in JCasC and Credentials - I think this needs a bigger refactoring. We should extract Vault Authentication mechanism in a common place and it should be used by both credentials and JCasC secrets code.