This patch trying to resolve mulitiple issues related to parameter
configuration:

Firstly, each DNN filters duplicate DNN_COMMON_OPTIONS, which should
be the common options of backend.

Secondly, backend options are hidden behind the scene. It's a
AV_OPT_TYPE_STRING backend_configs for user, and parsed by each
backend. We don't know each backend support what kind of options
from the help message.

Third, DNN backends duplicate DNN_BACKEND_COMMON_OPTIONS.

Last but not the least, pass backend options via AV_OPT_TYPE_STRING
makes it hard to pass AV_OPT_TYPE_BINARY to backend, if not impossible.

This patch puts backend common options and each backend options inside
DnnContext to reduce code duplication, make options user friendly, and
easy to extend for future usecase.

For example,

./ffmpeg -h filter=dnn_processing

dnn_processing AVOptions:
   dnn_backend       <int>        ..FV....... DNN backend (from INT_MIN to INT_MAX) (default tensorflow)
     tensorflow      1            ..FV....... tensorflow backend flag
     openvino        2            ..FV....... openvino backend flag
     torch           3            ..FV....... torch backend flag

dnn_base AVOptions:
   model             <string>     ..F........ path to model file
   input             <string>     ..F........ input name of the model
   output            <string>     ..F........ output name of the model
   backend_configs   <string>     ..F.......P backend configs (deprecated)
   options           <string>     ..F.......P backend configs (deprecated)
   nireq             <int>        ..F........ number of request (from 0 to INT_MAX) (default 0)
   async             <boolean>    ..F........ use DNN async inference (default true)
   device            <string>     ..F........ device to run model

dnn_tensorflow AVOptions:
   sess_config       <string>     ..F........ config for SessionOptions

dnn_openvino AVOptions:
   batch_size        <int>        ..F........ batch size per request (from 1 to 1000) (default 1)
   input_resizable   <boolean>    ..F........ can input be resizable or not (default false)
   layout            <int>        ..F........ input layout of model (from 0 to 2) (default none)
     none            0            ..F........ none
     nchw            1            ..F........ nchw
     nhwc            2            ..F........ nhwc
   scale             <float>      ..F........ Add scale preprocess operation. Divide each element of input by specified value. (from INT_MIN to INT_MAX) (default 0)
   mean              <float>      ..F........ Add mean preprocess operation. Subtract specified value from each element of input. (from INT_MIN to INT_MAX) (default 0)

dnn_th AVOptions:
   optimize          <int>        ..F........ turn on graph executor optimization (from 0 to 1) (default 0)
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
Reviewed-by: Wenbin Chen <wenbin.chen@intel.com>
Reviewed-by: Guo Yejun <yejun.guo@intel.com>