Feature Request: Allow input area in subscribe block to be styled and move to "Styles" tab

### What

When I work with the subscribe block and try and make the block stand out - it doesn't give me the option to change the colour of the input area and instead it takes the background of the group background.

In the panel there's:-

- [X] Button Background
- [X] Button Text
- [X] Border
- [ ] Input Background

I was also looking for these styles to be under the "Styles" tab - which they are not.

### Why

This will allow greater customisation of the block if the email input textbox could be customised.

### How

_No response_

Reproduce the subscribe block in the WordPress.com editor and inspect its block settings, especially the current controls and the Styles tab. The work is done when the input background can be customized independently and the relevant subscribe-block styles are available under Styles.

Written by the indexing model from the issue text.