Update arm velocity control function

[kabirkedia]

This change introduces support for arm velocity control on the Spot robot.

Reference: #104.
In the referenced implementation, the protobuf message was being reconstructed manually, which led to some issues. In this version, I simply copy the end_time field to the existing protobuf, avoiding the need to create a new one.

Based on the discussions in the PR, this is the approach @tcappellari-bdai preferred. It also enables us to leverage direct ROS-to-protobuf conversion without needing to explicitly map each field in a custom function.

I tested this thoroughly with cylindrical and angular velocity commands, and performed basic tests with cartesian velocity commands.

Related spot_ros2 PR: bdaiinstitute/spot_ros2#704

[khughes-bdai]

LGTM - thanks!

[coveralls]

Pull Request Test Coverage Report for Build 16350256683

Details

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 44.867%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spot_wrapper/spot_arm.py	0	2	0.0%

Totals
Change from base Build 15717190247:	0.0%
Covered Lines:	2181
Relevant Lines:	4861

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💛 - Coveralls