Allow access to ckpt_path within context of fit()

[speediedan]

What does this PR do?

This PR includes two minor changes that enable callbacks to restore checkpoints iteratively within the context of Trainer.fit() and prepares a deprecation path for the Trainer.{validated,tested,predicted}_ckpt_path attributes.

1. Changes BaseFinetuning.__apply_mapping_to_param_groups to protected instead of private
2. Deprecates Trainer.{validated,tested,predicted}_ckpt_path and replaces it with the read-only property Trainer.ckpt_path set when checkpoints are loaded via Trainer.{fit,validate,test,predict}

Fixes:
#10630

Does your PR introduce any breaking changes? If yes, please list them.

None

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Did you have fun?

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Make sure you had fun coding 🙃

[carmocca]

Awesome!

[ananthsub]

Great work!

[awaelchli]

Awesome, thanks! @speediedan picking up some speed!