Introduce PmLogString API

This API accepts JSON stringified key-value pairs, free-text and dumps it to system logs.
This API will be used by apps and nodejs services to log using pmloglib.

[GF-4464] Introduce log API to log stringified JSON key-value pairs and free-text

Open-webOS-DCO-1.0-Signed-off-by: Gayathri Srinivasan <gayathri.srinivasan@lge.com>
Open-webOS-DCO-1.0-Signed-off-by: Ameya Palande <ameya.palande@lge.com>

Change-Id: I2c1a78f6da6ec28ec161f2259d3db88761821c9a