Implement Authorization Callback Function

[henryluo]

Description

This feature introduces a callback function that allows service owners to define a fallback mechanism for access control within Knox. This capability supports the integration of custom logic to evaluate access decisions.

Changes Made

• Add new global accessCallback and SetAccessCallback that allows setting a callback function for fallback Access (ex. OPA)
• Add authorizeRequest function to wrap around principal.CanAccess, which additionally will execute the accessCallback when false is returned from principal.CanAccess
• Add a Raw() to all Principal types, which raw version (jsonable ID and Type) of all the principals.
• Modify TestNewFileClient to skip test if running on Linux and have the Knox daemon running

Testing

Run all tests in the repo, including new ones implemented for SetAccessCallback and authorizeRequest

Checklist

• I have run the code locally and it works as expected.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate).
• I have reviewed my own code and ensured it follows the code style of this project.

[jimmyl02]

I think overall this approach looks fine to me; I'm curious if we considered adding the authorize fallback function as a part of the PrincipalMux struct. That way we could keep the same CanAccess pattern and just modify the principalMux to fallback to this authorizer. Another benefit I see is that we don't have a global variable holding the fallback authorization function and instead it's bound to a struct which should be safer in terms of different pieces of code accidentally modifying the function