Optional expiration metadata for credentials with administrative warnings

### What feature do you want to see added?

The Jenkins Credentials Plugin currently provides a flexible and generic way to store credentials, but does not include lifecycle-related metadata such as expiration.

I would like to propose adding an **optional expiration date field** to credentials, along with **non-intrusive administrative warnings** in the UI.

### Proposed behavior:
- Allow setting an **optional expiration date** when creating or updating a credential
- Display a **warning in the Jenkins UI** when:
- the expiration date is approaching (configurable threshold)
- the credential is expired
- Make this information available as **read-only metadata via existing APIs**

### Design considerations:
- **No enforcement**: expired credentials remain usable
- **No change in runtime behavior** (lookup, binding, pipelines, etc.)
- **Fully optional** and backward compatible
- **No performance impact** (UI-level feature only)

### Goal:
Provide better **visibility for administrators** regarding credential lifecycle, without introducing policy enforcement or altering the plugin’s core behavior.

### Upstream changes

_No response_

### Are you interested in contributing this feature?

_No response_