lib: qmi_encdec: Add support for string data type

Add support to encode and decode string data type into a QMI message.
If a string has a <Type>:<Length>:<Value> information associated with it,
then the length information holds the length of the string. If the string
is part of a nested structure, then the length of the string is explicitly
encoded into the QMI message.

Change-Id: If488d25eaf6d97cd9422848e7bfdfd17cdb68fa0
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>