Add new function for a synthesis parameter

Add new function for a synthesis parameter

- Requirements:
The app needs to play each utterance with different configuration such
as pitch, speed, volume, or background volume.

- Contents:
This patch adds new function for a synthesis parameter. The synthesis
parameter is newly introduced by this patch. Using current API, the
client can only change the speed value using fixed parameter of
tts_add_text() function. The synthesis parameter is a kind of handle, so
client can set various configuration value using provided setter
functions. The client can manages the synthesis parameter with related
functions and these functions will be expandable.
Thus, through this change, TTS framework will provides changable
configuration for each utterance.

Change-Id: I7c15d72e253e047b45ec03424e4a729d95007843
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>