Add an interface option to let users set empty_trash_days

[mrfoxtalbot]

Most users are unaware of the empty_trash_days settings or how to change it. This can sometimes make them lose their content if they trash their posts instead of setting them as drafts.

In #61511 I suggested we could display the value of empty_trash_days to minimize confusion but we could go further and introduce a setting to allow users to set this value via their dashboard.

Trac Ticket - Core-61598

## Summary

Fixes an issue with the EMPTY_TRASH_DAYS value, right now user can't manually set the EMPTY_TRASH_DAYS value
from the backend, so this PR addresses the issue and adds a settings field under Writing, which lets the user set the EMPTY_TRASH_DAYS value.

## Description

### Problem

Right now, the user can't manually set the EMPTY_TRASH_DAYS value from the backend, neither all of them are aware of the process on how to do it. They either need to hire a developer to do the changes or need to manually change it.

### Solution

To resolve this issue, I have made the following changes:

• Created a Settings field under Writing
• The default value will always be 30, if the user enters any value, the EMPTY_TRASH_DAYS value would get updated as well.
• This solution provides a simple UI for the End Users who are not technically sound enough to increase or decrease the EMPTY_TRASH_DAYS value from the backend.

### Changes Made

• Sanitize the keys of the $_REQUESTautofocus? array using sanitize_key().
• Sanitize the values of the $_REQUESTautofocus? array using sanitize_text_field().
• Combine the sanitized keys and values into a new array using array_combine().

@Debarghya-Banerjee left smaill suggestion. Overall LGTM.

@Debarghya-Banerjee left small suggestion. Overall LGTM.

Hi @itzmekhokan, I have fixed the above requested changes, and have assigned you for re-review again. Thanks :-)