client/isolate: add --dump_json support to exparchive
client/isolate: add --dump_json support to exparchive

This uses the same flag name and JSON format as the batcharchive
command, but will only ever emit a single digest entry.

R=mcgreevy@chromium.org
BUG=598990

Review-Url: https://codereview.chromium.org/2584323002

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go