Added support for custom parameters in subclasses of SaveConfigCallback

[mauvilsa]

What does this PR do?

Implements suggestion in #14628 (comment). Does not close that issue, but it makes it possible for people to implement by themselves.

This has the benefits that:

• We remove duplications of parameters that were in both SaveConfigCallback and in LightningCLI.__init__.
• Makes it possible for people to subclass SaveConfigCallback adding new parameters, and allow these be given on LightningCLI instantiation.

Currently the save of the config is not even mentioned in the docs. The pull requests #14976 adds it. Could be that this is added to the docs in that pull request.

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

SaveConfigCallback parameters in LightningCLI instantiation must now be given in save_config_kwargs, and making save_config_kwargs, save_config_overwrite and save_config_multifile deprecated.

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