[ros_speech_recogniton, respeaker_ros] add confidence field

[knorth55]

this PR adds confidence field in ros_speech_recognition and respeaker_ros`.

current master branch does not return confidence field, which causes error in some nodes.
this PR adds 1.0 (because there is only one candidate) in confidence field.

$ rostopic echo /speech_to_text
transcript:
  - konichiwa
confidence: [1.0]
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