feat: allow immutable file upload for wandb logger

[cgebbe]

What does this PR do?

Current Problem

• given wandblogger with log_models=True and e.g. one ModelCheckpoint with save_top_k=3
• when finalizing a run
• then many models will be uploaded to wandb and beforehand copied to a local temporary directory
• and this copying can easily lead to a full disk space and thus a failed run.

Solution

• Allow to specify immutable as policy for add_file to prevent copying.

Open questions

• I didn't add a test, since the wandb logger is a stub and this feels like a very safe modification. Is that okay? Otherwise, what do you suggest?

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📚 Documentation preview 📚: https://pytorch-lightning--20193.org.readthedocs.build/en/20193/