Split PolicyChecker from PolicyManager

[prdoyle]

PolicyManager has a number of responsibilities, including at least:

• Determining the entitlements granted to a given requesting class
• Two dozen check methods that determine whether some required entitlement is present

This PR splits those into two objects, with the immediate consequence being that we can now swap in a PolicyManagerForTesting that modifies the behaviour.

With this change, the design looks as follows:

• The EntitlementChecker interface has one method per sensitive JDK method. Its implementation (called ElasticsearchEntitlementChecker) determines which kind of check we should do for each method. Its methods are all one-liners that call PolicyChecker.
• The PolicyChecker interface has one method per distinct kind of check (about 27 methods in total). Its implementation (PolicyCheckerImpl) has the job of determining the caller class, querying the PolicyManager to determine what entitlements are granted to that class, and then checking whether the required entitlements are present.
• The PolicyManager class determines, for a given caller class, what entitlements that class has been granted, and whether it is trivially allowed.

It's PolicyManager that implements getEntitlements and isTriviallyAllowed. The idea is that we'd create a PolicyManagerForTesting that would override these to provide the required semantics when running in junit.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)