Feature Request: Paid Content Block Styling - Lock Login button or provide allignement options

### Impacted plugin

Jetpack, mu wpcom

### What

The goal is to provide the users a better way to stylize their Paid content Buttons or communicate that it's not an option.

### How

The Subscribe button is locked (Can't be moved or removed), which is nice. But the Login button isn't, which allows users to endup in unexpected situations.
One of those situations is when they move the Login button outside of the "Premium Content buttons", it's the only direct way to make the buttons align vertically instead of horizontally. This customization is shown here:

The issue with this is that some of the login button stylings is inherited from the "Premium Content buttons", and having the button outside of its parent gives unexpected results (see width issue here):

The ideal fix here would be to add alignment options on the "Premium Content buttons" similar to [the default Buttons block](https://wordpress.org/documentation/article/buttons-block/).
But a quick way to prevent user confusion is to lock the Login button from being moved in the first place.

No files or tests are named. Start by locating the Paid Content block's Premium Content buttons and its Subscribe and Login button controls, then determine whether alignment options or locking the Login button best addresses the reported styling problem. Done means the buttons no longer produce unexpected layouts and the chosen behavior is clear to users.

Written by the indexing model from the issue text.