Add option to change "=" symbol in ModelCheckpoint filenames

[giorgioskij]

What does this PR do?

Part of #17825

This PR adds a feature, as discussed in issue #17825, to change the default "=" symbol between the name of a field and its value in the filenames generated by the ModelCheckpoint class. There was already an option to change the separator symbol in the filenames, so this follows the same exact implementation style.

Example:

ModelCheckpoint.CHECKPOINT_JOIN_CHAR = "-"  # this was already present
ModelCheckpoint.CHECKPOINT_EQUALS_CHAR = ":"  # this is my addition
ckp = ModelCheckpoint(dirpath=".", monitor="valid_loss")
trainer = L.Trainer(callbacks=[ckp])

now this trainer will save its checkpoints as: epoch:1-step:1234.ckpt instead of epoch=1-step=1234.ckpt

While adding the new attribute to the docstring of the class ModelCheckpoint, we noticed a discrepancy between code and docstring. The docs say:

For example, you can change the default last checkpoint name by doing
``checkpoint_callback.CHECKPOINT_NAME_LAST = "{epoch}-last"``

However, the correct way to change that attributes at the moment is by using the class name, and not the instance name, so:
ModelCheckpoint.CHECKPOINT_NAME_LAST = {epoch}-last

In issue #17825 we decided to keep this as is, and make a new pr to change the code to behave how it is currently documented.

I also updated CHANGELOG.md to mention this new attribute, including the link to this PR.

Thanks a lot for your time reviewing this!

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[awaelchli]

Nice addition!
We would need a test too. I suggest to extend the existing test_model_checkpoint_format_checkpoint_name in test_model_checkpoint.py.

[carmocca]

LGTM!