[ofono][rilmodem] OemRaw interface and rilplugin implementation

Signed-off-by: Tommi Kenakkala <tommi.kenakkala@oss.tieto.com>

Introduces an OemRaw interface and a riplugin implementation to match it.
Functionality is a "pipe" between ofono interface and RIL interface passing client's byte arrays to RIL and responses to client.