Allow `extra_epochs` flag in `Trainer.fit` to control finetuning time

Nobody has claimed this yet.

## 🚀 Feature

`Trainer(max_epochs=100).fit(model, train_dl, ckpt_path=ckpt_path, extra_epochs=True)` would finetune for 100 epochs

### Motivation

Finetuning for N epochs requires knowing the previous number of epochs M and setting `Trainer(max_epochs=M+N)`. Google did not tell me how to achieve this.

### Pitch

Finetuning training time or number of epochs should be configurable.

### Alternatives

Setting many epochs and manually stopping

### Additional context

It would be cool with `max_time` too. I hope this is already solved and this issue is unnecessary.
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cc @justusschock @kaushikb11 @awaelchli @borda @rohitgr7

Start at the Trainer.fit entry point and inspect how ckpt_path restores the previous epoch and how max_epochs is interpreted. Define the expected behavior for extra_epochs=True, including the max_time idea only if it is in scope, and add or update tests showing that finetuning runs for the requested additional epochs.

Written by the indexing model from the issue text.