Added method for spawning multi-asset

[Kyu3224]

Description

Regarding issue #3070, I added support for spawning objects in each environment within a multi-USD (or asset) setup using various methods. Users can define custom functions in wrappers/utils.py or any other desired location, specify them in the config, and have them dynamically loaded using hydra.utils.get_method. The loaded choice_fn will then iterate through all prim_paths and assign asset indices based on the defined rules.

Following @Mayankm96 's advice, this design allows for user-defined logic to determine how assets are distributed across environments, providing both built-in methods (e.g., sequential, split) and the flexibility to extend with custom strategies.

I aimed to align this as closely as possible with the Isaac Lab code style and direction, but I'm not sure if it worked well, so I would appreciate your feedback. Basically, I have implemented sequential and split methods, so users can select between these two by default, and they can also add and use additional methods as needed.

Type of change

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

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