ohos: Implement rate in qt speech engine

Store rate in backend-specific scale (0,5..2) internally while exposing
Qt standard range (-1..1) in API.
Rate conversion:
Qt      Tts
1.0     0.5     min rate
0.0     1.0     normal rate
1.0     2.0     max rate

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: Iecd0031103f7ed002014de66c55274812478e7d7
Reviewed-by: Zbigniew Chyla <zbigniew.chyla@qt.io>
Reviewed-by: Liang Qi <liang.qi@qt.io>