improves recorder manager to support customized demo index

[rebeccazhang0707]

Description

Current manager-based workflow leveraged recorder_manager to store demo obs and actions to hdf5 file.

• But it only outputs successful demos and accumulates the demo index
• Not supporting customized demo index (the use case is like: replay an existing hdf5, and store successful demos while keeping its original demo index)

The modification in this PR:

• keeps original function, but extends to store demo with specific demo index

Type of change

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

Screenshots

Before	After

Checklist

• I have read and understood the contribution guidelines
• 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

[ooctipus]

@peterd-NV I think this requires your review as well : ))