Always set data_key

Hello all,

in my application we use and love marshmallow. We have a lot of dynamic coding, therefore we access fields e.g. over the `declared_fields` attribute. Also we need most of the time the `Field.data_key` attribute which is defined as `str | None`.

I would like to suggest an option, which always sets the data_key so that it will never be None. If the user provides it, fine. Else, just set it to the field name. I guess this could also simplify some coding, since you can expect that this attribute is always set.

What do you think?

Start by reading the Field.data_key definition and checking how fields are exposed through declared_fields. Determine the expected behavior when a field name is used without an explicit data_key, then inspect the existing field tests to identify where this option and its compatibility should be covered.

Written by the indexing model from the issue text.