[Nullable Context] Outstanding issues

[schmittjoseph]

While enabling nullable context throughout the code, we've found several places that need more attention:

• EventPipeProvider.Keywords
• StorageOptions.DumpTempFolder
• ProcessFilterDescriptor.Value
• CollectionRuleOptions.{Filters,Actions}
• ICollectionRuleTriggerOperations.TryValidateOptions
• ActivityExtensions.GetSpandId