Fixes ObservationManager history buffer corrupted by external calls to ObservationManager.compute (#2885)

# Description

When observation group has history length greater than zero, calling
`ObservationManager.compute` modifies history state by appending current
observation to history. This creates history corruption when
non-`ManagerBasedEnv` classes invoke `ObservationManager.compute`. This
PR introduces `update_history` flag (default to `False`) and only
`ManagerBasedEnv` has the privilege to run `ObservationManager.compute`
with `update_history=True`. If `update_history=False` and the history
buffer is `None`, a copy of history is returned instead of the original.

I have added test cases to verify this fix is effective.

Fixes #2884 

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

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

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Signed-off-by: ooctipus <zhengyuz@nvidia.com>
Signed-off-by: Kelly Guo <kellyg@nvidia.com>
Co-authored-by: ooctipus <zhengyuz@nvidia.com>
Co-authored-by: Kelly Guo <kellyg@nvidia.com>