Feature Request: Donations Block Add Support for Causes

### What

In the donations block, add the ability for the user to add various "causes" so that visitors to the site that are donating can decide to which cause they want to give money. This could be a simple dropdown that is visible to the visitor that they could select and then on the payments screen, it would add a note so that the user knows where to put this money.

### Why

As an example, in this chat, 34887591-hc the user works at a church and would like to add the ability for visitors to the site to be able to decide to which "cause" they would like to donate money. This is not possible with the donation block currently, so to achieve this the user would need to put multiple donations blocks on the same page, one for each "cause". This user ultimately decided to use the WooCommerce plugin which does have this feature. This user does not really need all of the features and complexity of WooCommerce and only wants to add a simple donation option to their site.

https://woocommerce.com/products/donation-product-for-woocommerce/

For sites that accept donations, I believe it is fairly common for those organizations to have multiple causes that they are supporting and currently there is not a way for contributors to alert these organizations to where they would like the money to go when they donate.

The donations block is so easy to set up, especially with the ability for visitors to choose a custom amount, and this feature would add even more versatility.

### How

We could add another toggle that would add in the ability to add text inputs in the Donations Block.


Then, when someone completes a donation, on the payments screen, we could show a note that indicates which "cause" the contributor selected.

Start at the Donations Block entry point and trace how the selected donation amount reaches the payments screen. Review the proposed cause selector and the existing payment-note flow, then clarify how causes should be configured and carried through. Done means donors can select a cause and the selected cause is shown in the payment note.

Written by the indexing model from the issue text.