Implement compiler warnings

[charithe]

Currently the compile stage only produces errors -- which cause the policy to be excluded from execution. However, there are certain problems that don't necessarily make the policy invalid (e.g. in a scoped policy set, each policy has a different schema reference). We should have a way to produce these warnings but nevertheless continue working as normal.

[haines]

Another case where a warning would be useful is deprecating CEL functions (https://github.com/cerbos/cerbos/issues/669).