feat(rviz): make it possible to set default topic name

[satoshi-ota]

Description

I fixed logic so that we can set default topic name in rviz config like this:

  - Class: tier4_string_viewer_rviz_plugin::StringViewerPanel
    Name: StringViewerPanel
    topic: /planning/scenario_planning/lane_driving/behavior_planning/behavior_path_planner/debug/internal_state

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Tests performed

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Interface changes

Effects on system behavior

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[kyoichi-sugahara]

LGTM!