add ValidateKey method to comparer

[RaduBerinde]

We have an Experimental.KeyValidationFunc to validate keys but it is only used in one case (check compacted file bounds). We should promote this to a method on the Comparer and use it more, especially on API calls like Download, Compact, IngestExternalFiles, IngestAndExcise. It is preferable to error out a specific operation than letting an invalid key go through and then either crash later or do something incorrectly.

See cockroachdb/cockroach#128043 for a few cases where CRDB was passing keys that were not properly encoded.

Jira issue: PEBBLE-233