enhancement: allow VBR encode min-bitrate to be specified via user properties

Currently, gst-vaapi treats user-specified bitrate setting as max-bitrate and hard-codes min-bitrate = bitrate * 0.70 (i.e. target percentage = 70%).

In gst-msdk we are able to specify min-bitrate and max-bitrate via bitrate and max-vbv-bitrate properties, respectively.

Please add similar capability in gst-vaapi to allow user to specify target-percentage (or min-bitrate) property.