SpikeGadgetsRawIO: detect timestamp gaps and expose acquisition timestamps (impl of #1773)

[h-mayorquin]

I have been working with SpikeGadgets and I realized that this format also has timestamps in each packet's header (4-byte uint32, see the packet diagram in spikegadgetsrawio.py), but SpikeGadgetsRawIO does not currently read it. We should use it to run an integrity check on parse (like the one added to IntanRawIO in #1484) and to expose the per-packet hardware timestamps via a get_spikegadgets_timestamps() method (parallel to #1652 for Intan). This is the SpikeGadgets-specific implementation of the general gap-handling API in #1773.