Allow user to specify a directory for extracted pcap flows

[philrz]

Right now when the user clicks the Packets button, the flow extracted from the larger pcap gets stored in a file in a tmpdir before being opened in whatever app the OS has tied to that file type. In a group discussion on 2021-02-02, we recognized that it might be helpful to allow the user to specify a specific, non-tmpdir folder. A use case that might be enabled by this would be the saving of several separate extractions to the same folder, such as if data is being accumulated as part of an investigation. This might make sense as an option in the Preferences menu, or perhaps something that could be specified via right-click on the Packets button, possibly with memory of recent folder choices to make it easy for the user to keep adding to a particular location.

[philrz]

It looks like the path to the tmpdir is obtained here:

https://github.com/brimdata/brim/blob/1ee21a85330f62430d65298f61939e95469dd1a5/src/plugins/brimcap/brimcap-plugin.ts#L225

Therefore once the setting is added to Preferences this would be the spot to make use of it.

[philrz]

Verified in Zui commit 2d23f20.

As shown in the attached video, by default pcap slices are still stored in $TMPDIR. However, a new setting is now available in Settings > Packet Captures > Folder For Extracted pcaps that allows the selection of an alternate location. When this setting is populated, pcap slices are stored there instead. If the setting is cleared, they once again are stored in $TMPDIR.

Verify.mp4