CoW: add warning mode for cases that will change behaviour
xref https://github.com/pandas-dev/pandas/issues/56019, https://github.com/pandas-dev/pandas/issues/48998

An initial PR working towards one of the remaining TODO items:

> Add a warning mode that gives deprecation warnings for all cases where the current behaviour would change (initially also behind an option) 

The current PR adds an option `pd.options.mode.copy_on_write = "warn"`, and then everything behaves as the current default (without CoW enabled) but will raise a warning in case the behaviour will change if CoW would be enabled.

As a first step, this PR adds the option, and adds the warning in one specific code path that can trigger CoW, i.e. updating a Series (`Manager.setitem_inplace`). So this covers a typical case like:

```
s = df["col"]
s[0] = ..
```

no longer updating `df`.

In addition, I also turned off SettingWithCopyWarning when the CoW warning mode is enabled (because in that case, those warnings are no longer correct for the future behaviour, and it seemed better to not raise both kinds of warnings at the same time)
There are still a whole bunch of failures (with the warning mode enabled, for which I still need to add a CI build), but quite some of them are due to inplace operators (eg `s += 1`) unnecessarily triggering CoW, which is something I will try to tackle as a precursor. @phofl I removed the last commit about the chained assignments again, and moved that to a separate PR (https://github.com/pandas-dev/pandas/pull/55522). So this PR then just focuses on adding the infrastructure, and the warnings for (non-chained) series setitem (the things going through `setitem_inplace`). Then once this is merged, I can work more easily in parallel on different areas. I added a `tm.assert_cow_warning` helper function to use for all the cases where specifically checking for this warning.

@phofl @mroeschke if possible, I would like to get this merged shortly if possible, as there is a lot more work to do here, but that will be easier done in parallel once this base PR is in. I’ll check later today or tomorrow  thx @jorisvandenbossche 