Add a "duration" mode for event term

[luoye2333]

Description

The original event term apply_external_force_torque is working throughout the episode of the robot/env, which is not useful in training, so a new event term is added: apply_external_force_torque_duration. It applies force/torque to the robot just for a while and then cancel. The original implemeted modes of event term "interval" cannot do this, so a new mode is added called "duration".

The changes have been tested in train/play and work as wanted.

Type of change

• New feature (non-breaking change which adds functionality)

Screenshots

Please attach before and after screenshots of the change if applicable.

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there