ExtensionArray.interpolate() method and tests
- [ ] 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).
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- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Adds a default interpolate method for numeric EAs.

Partially fixes #40252 by removing fill_value from interpolate signiture. This doesn't fully close as I haven't implemented interpolate for integers - should interpolating an IntegerArray return a IntegerArray or FloatArray? 

Is this the right way to go about testing interpolate? I couldn't think of a simpler way to go about it.

This works for Series(EA).interpolate(), but falls over for DataFrame(EA).interpolate() as axis=1 is passed. 
This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale and the checks are all not passing yet. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.