Enable heterogeneous equality within the runtime.

The type-checker will raise errors when cross-type comparisons are made within strongly typed
expressions; however, for dynamically typed data such inferences are not possible meaning they
must be handled appropriately at runtime.

Heterogeneous equality is being introduced to ensure that CEL and proto equality are better
aligned, that CEL's notion of runtime equality is consistent with Zeta SQLs, and to make it easier
to work with dynamically typed data: e.g.json.string_value != 2

This change is a semantic change which has not yet been reflected in the cel-spec but will be
soon. The change is generally regarded as safe as it shifts error states to non-error states, but
in very rare circumstances it is possible that an application may rely on this error to be produced.