Add codec control for external rate control lib

VP9E_SET_EXTERNAL_RATE_CONTROL
One can assign an external library using the control flag,
VP9E_SET_EXTERNAL_RATE_CONTROL.
The args alongside the control flag should be of type char**.
args[0]: char* points to the path of rate control library
args[1]: char* points to the config of the rate control library.

Change-Id: Iae47362cdfafa00614bac427884bffcf6944c583
3 files changed