decklink: Add support for extracting/embedding KLV metadata in HD-SDI

This is an upcoming need, but thought I'd document it here. HD-SDI (SMPTE 292M) can include KLV metadata in the VANC/VBI (and sometimes HANC), as described in SMPTE ST 291 and RP 214. This would need a GstMeta for KLV, for which there is already a merge request.