[Policy Compiler] Add optional description fields to config and policy variables

[l46kok]

https://github.com/google/cel-go/blob/master/policy/parser.go#L252
https://github.com/google/cel-go/blob/master/policy/config.go#L111

Similar to how we support adding descriptions in various places of the policy engine, affixing variables with custom description is an often requested feature.