[validate] Flag to treat warnings as errors or a validator configuration file

[Vilsol]

Currently the validator spits out warnings, but there is no way to treat them as a failure, as the command then exits with a 0 exit code.

It would be extremely useful if there was either a flag to treat warnings as errors, or to have a custom configuration file which would specify what validator rules should count as failures.

There is also a confusing example in the playground. The "Google IAM in SpiceDB" example shows that you should point a permission to a relation, but your own validator spits this out that it's a warning, and you should point to a permission instead?

(continued from https://canary.discord.com/channels/844600078504951838/844600078948630559/1282142088061911131)