Merge pull request #18535 from joshdoe:gray16_gstreamer_writing

Merge pull request #18535 from joshdoe:gray16_gstreamer_writing

Add CV_16UC1/GRAY16_LE support to GStreamer backend for VideoWriter

* videoio(backend): add Writer_open_with_params to plugin API

This will allow arbitrary parameters to be passed to plugin backends

* videoio(gstreamer): add GRAY16_LE/CV_16UC1 writing support to GStreamer

This introduces a new property VIDEOWRITER_PROP_DEPTH, which defaults to
CV_8U, but for GStreamer can be set to CV_16U.

Also, fix another test to not fail if plugin isn't found, copying logic
from the read_write test.

* videoio(plugin): fix handling plugins with previous API level

* videoio: coding style

* fix warning