Provide a mechanism for surfacing highlights from attachments

[NickEntin]

This stems from a desire to have bug reporters (in particular the ARKEmailBugReporter) treat the log store as just another attachment, rather than special casing it. Currently the only major behavioral difference between the log stores and the other attachments is that the reporter pulls the recent errors out of the logs and surfaces them at the top level. This is vital behavior in the case where attachments aren't available, but it's also very useful when there are attachments to provide a quick hint as to what the problem might be before digging into each attachment.

I think it would be fairly straightforward to introduce a highlightsSummary property (exact naming TBD) to ARKBugReportAttachment so that any attachment could optionally surface highlights at the top level. This would also unblock making the log store be a regular attachment, since the recent error logic could be easily moved to the LogStoreAttachmentGenerator.