Payment Button Block: Create a custom amount variation

### Impacted plugin

Jetpack

### What

Allow users to setup a custom amount Payment Button block.

### How

This is an optional follow up to https://github.com/Automattic/jetpack/issues/23416.

Our infrastructure already supports custom amount products, so for starters, we can just rely on the payment form popup.
In the long term, we might want to create a block variation that handles this!

Start by reviewing issue #23416 and the existing payment form popup and custom amount product infrastructure described here. Determine the initial block behavior that can rely on the popup, and confirm that a Payment Button block can expose a custom amount option; the long-term block variation remains a separate design question.

Written by the indexing model from the issue text.