Subscribe block: allow modifying confirmation modal

### Impacted plugin

Jetpack

### What

Allow modifying confirmation modal

### How

Allow modifying confirmation modal's message and modal itself:

We can use similar mechanism as with [another subscription modal](https://github.com/Automattic/jetpack/pull/31393) (peKye1-5H-p2); a filter initially, and template part when they want to edit it.

Start by reading the subscription modal implementation referenced in pull request 31393 and compare it with the Subscribe block confirmation modal shown in the screenshot. The work is done when the modal message and modal markup can be modified through the proposed filter and template-part mechanisms, with the relevant behavior verified in the block.

Written by the indexing model from the issue text.