(See {T292868} for a similar task involving boolean flags.)
ParserOutput contains a number of "accumulator" methods, which add new values to a list. The creation of the Content-Security-Policy source lists (`::addExtraCSP*Src()`) are good examples, but we also have some TTL maintenance operations like `updateRuntimeAdaptiveExpiry()` which funamentally maintain a list of possible "maximum time to live" values, and then selects the lowest of these in when `::finalizeAdaptiveCacheExpiry()` is finally called.
In this task, create a new `::addOutputData()` method to generalize the accumulators for Parsoid's `ContentMetadataCollector` interfaces so that this functionality is exposed to extensions but the full complexity is not exposed to Parsoid itself. T292868 is similar, but that task deals with boolean-valued flags. In order to maintain type safety, we'll keep the string/int accumulator methods separate from the boolean flag methods.