gst-launch: Add switch to enable user to force a clock on the pipeline

Sometimes, one wants to force a clock on some pipelines - for instance, when testing TSN related pipelines, one usually uses GstPtpClock or CLOCK_REALTIME (assuming system realtime clock is in sync with network one). Until now, one needs to write an application for that - not difficult, but quite boring if one just wants to test something. This patch adds a simple way to force some clocks to be used by the pipeline: switch -k.

Three clocks supported now: 'realtime', 'monotonic' and 'ptp'. The two first are actually the GstSystemClock, but defining its type to GST_CLOCK_TYPE_REALTIME and GST_CLOCK_TYPE_MONOTONIC, respectively. The 'ptp' one is GstPtpClock, using domain 0.

Not using the switch preserves current behaviour, in which pipeline elects its own clock depending on elements.