ENH: EA.fillna copy=True
- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

cc @phofl for comments on the CoW handling in Block code
cc @jorisvandenbossche for thoughts on deprecation path for 3rd party EAs

Edit: also worth discussing the "author's discretion" on respecting copy=False.
Updated to ignore "copy" in the base class fillna method in ffill/bfill cases.

Added a warning+test for 3rd party EAs that dont yet have the copy keyword in the signature. i think comments have been addressed Sorry I thought that we had already merged this. Will merge when CI is green thx @jbrockmendel 