Explicit actuator gain randomization

[MoreTore]

Description

resolves: #1000
resolves: #1011
depends: #1006

Type of change

• New feature (non-breaking change which adds functionality)
• This change may require a documentation update

Checklist

• have run the pre-commit checks with ./isaaclab.sh --format
• My changes generate no new warnings
• I have added my name to the CONTRIBUTORS.md or my name already exists there
• I have made corresponding changes to the documentation
• 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

[MoreTore]

Can I have some guidance on how the test should be completed

[MoreTore]

in articulation.py we could set the default_joint_stiffness and default_joint_damping properly , instead of settings all zeros for explicit actuators. this would simplify this PR.
but for example in source/extensions/omni.isaac.lab/omni/isaac/lab/assets/articulation/articulation.py:

...
set defaults from the sim first
...


for actuator_name, actuator_cfg in self.cfg.actuators.items():
  if isinstance(actuator, ImplicitActuator):
     self.write_joint_xxx
     ...
  else:
     self.write_joint_xxx
     ...
     # over write the defaults we got from the sim with our local data
     self._data.default_joint_stiffness[slice(None), actuator.joint_indices] = (
        actuator._parse_joint_parameter(actuator.cfg.stiffness, actuator.stiffness)
     )
     self._data.default_joint_damping[slice(None), actuator.joint_indices] = (
        actuator._parse_joint_parameter(actuator.cfg.damping, actuator.damping)
     )

[MoreTore]

Added my proposed changes to articulations.py to this PR.