fix(pointcloud_preprocessor): change point cloud conversion logic

[0x126]

Related Issue(required)

Description(required)

This PR changes

• Fix the method of converting the type of pointcloud before concatenating
• Change the function name

Review Procedure(required)

• Play rosbags and make sure the concatenated pointcloud is output.

Related PR(optional)

https://github.com/tier4/autoware.iv/pull/1912

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[1222-takeshi]

LGTM