ohos: Implement pitch in qt speech engine

Store pitch in backend-specific scale (0,5..2) internally while exposing
Qt standard range (-1..1) in API.

Pitch conversion:
Qt      Tts
1.0     0.5     min pitch
0.0     1.0     normal pitch
1.0     2.0     max pitch

Cache the scaled value. The cached rate will be used later when speak
method will be called.

Task-number: QTBUG-147179
Pick-to: 6.12
Change-Id: I75f5022cecce29f328a3e320efc156c7e6765a81
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>