Add "encoder" parameter to RTMP Sink to support H.264 software encoding

[rjhowell44]

Previously, the RTMP Sink was only able to support hardware H.264 encoding. Therefore, the Encoder parameter was omitted from the constructor and was fixed internally.

Now that DSL is to support software encoding, the parameter needs to be added. The choice of encoder will be limited to
DSL_ENCODER_HW_H264 and DSL_ENCODER_SW_H264

More research needs to be done to find plugins that support other formats.