Add withConfirmation(ValueProvider<T, String>) for per-item confirmation messages

[javier-godoy]

Feature proposal

EasyRowAction.withConfirmation(...) currently accepts static strings only:

public EasyRowAction<T> withConfirmation(String message)
public EasyRowAction<T> withConfirmation(String title, String message)

There is no way to derive the confirmation text from the row item, so the most common phrasing for a
destructive action — "Are you sure you want to delete John Smith?" — cannot be expressed. The
message has to stay generic ("Are you sure you want to delete this item?"), or the developer has to
abandon withConfirmation altogether and open a ConfirmDialog by hand inside the action handler.

Rolling your own dialog means losing what EasyRowAction already does for you:

• the confirmPending guard that stops dialogs from stacking on rapid clicks;
• re-checking visibleWhen / enabledWhen at confirm time, not just at click time;
• the Vaadin 24 / 25 compatibility shim around ConfirmDialog.addOpenedChangeListener.

This is the same gap that was already closed for tooltips, which offer both a static and a
row-derived form (tooltip(String) / tooltip(ValueProvider<T, String>)).

Describe solution expectations

Add ValueProvider overloads mirroring the tooltip pair:

public EasyRowAction<T> withConfirmation(ValueProvider<T, String> messageProvider)
public EasyRowAction<T> withConfirmation(String title, ValueProvider<T, String> messageProvider)
public EasyRowAction<T> withConfirmation(ValueProvider<T, String> titleProvider,
                                         ValueProvider<T, String> messageProvider)

Usage:

grid.addRowAction("Delete", VaadinIcon.TRASH, this::delete)
    .withConfirmation("Delete person",
        person -> "Are you sure you want to delete " + person.getFullName() + "?");

The three signatures do not collide with the existing (String) / (String, String) pair, so this
is source- and binary-compatible.

Implementation looks small, because the dialog is already built lazily at click time:

• confirmDialogSupplier (a SerializableSupplier<ConfirmDialog>, field declared at
  EasyRowAction.java:129) is only invoked from execute(T item), where the row item is already in
  scope. Changing it to a SerializableFunction<T, ConfirmDialog> and calling .apply(item) instead
  of .get() is enough to make the whole dialog item-aware.
• The private builder at EasyRowAction.java:209 becomes provider-based, and the existing String
  overloads delegate to it through Constant.of(...) — exactly the way tooltip(String) delegates
  today.
• null title should keep meaning "no header", as it does now via
  withConfirmation(String message) → withConfirmation(null, message).
• ValueProvider is Serializable, so this stays consistent with the add-on's serialization
  requirements.

Whether the confirm/cancel button labels should also become row-derived is probably not worth it —
they are not item-dependent in practice — but the private 4-arg builder would support it for free if
that changes.

Additional information

References in src/main/java/com/flowingcode/vaadin/addons/easygrid/actions/EasyRowAction.java
(master):

What	Line
tooltip(String) — the precedent, delegates via Constant.of	170
tooltip(ValueProvider<T, String>) — the precedent	182
withConfirmation(String message)	194
withConfirmation(String title, String message)	205
private withConfirmation(title, message, confirmText, cancelText) builder	209
confirmDialogSupplier field	129
execute(T item) — where the dialog is built, item in scope	270

RowActionsDynamicDemo would be the natural place to show a per-row confirmation message alongside
the existing dynamic tooltip / visibleWhen / enabledWhen examples.