Capture log file mtimes in support bundles

[wfchandler]

Currently the mtime for log files captured in support bundles is always set as the default value of 1980-01-01T00:00. To more easily sort files by time, it would be convenient to capture their actual mtimes in the bundle zip.

Attempt to get the mtime for logs captured by sled-diagnostics, falling back on the default if this fails, and copy the mtime into the final bundle zip in nexus.

[papertigers]

I think this is good information to pass along in the zip file, however I believe we already have access to the mtime from oxlog, if you look at get_zone_logs_inner() we are grabbing <LogFIle>.path instead of using the LogFile type given to us directly from oxlog. It's probably better if we thread that through everywhere since we already paid for the stat call already and have the data. I realize that's pretty much a complete rework of this PR but is more efficient, thoughts?

[wfchandler]

I think this is good information to pass along in the zip file, however I believe we already have access to the mtime from oxlog, if you look at get_zone_logs_inner() we are grabbing <LogFIle>.path instead of using the LogFile type given to us directly from oxlog. It's probably better if we thread that through everywhere since we already paid for the stat call already and have the data. I realize that's pretty much a complete rework of this PR but is more efficient, thoughts?

Great point, I've updated to use the cached mtime when available.