added expiry date in license

[scarlet2131]

Description

Adds expiry_in_days field to _license API response for easier license monitoring

This PR implements the enhancement requested in issue #129726 to add an expiry_in_days field to the _license API response. This field shows the number of days until license expiry as a float (e.g., "30.2"), making it much easier for users and monitoring tools to check license status without manual date calculations.

Changes Made

• Added getDaysUntilExpiry() method in LicenseUtils.java to calculate days until license expiry
• Added EXPIRY_IN_DAYS field constant in License.java
• Enhanced toXContent() method in License.java to include the new field in JSON response
• Added comprehensive unit tests in LicenseUtilsTests.java covering all scenarios

Example API Response

Before:

{
  "license": {
    "status": "active",
    "type": "trial",
    "expiry_date": "2025-07-19T22:05:12.332Z",
    "expiry_date_in_millis": 1542665112332
  }
}

After:

{
  "license": {
    "status": "active", 
    "type": "trial",
    "expiry_date": "2025-07-19T22:05:12.332Z",
    "expiry_date_in_millis": 1542665112332,
    "expiry_in_days": "30.2"
  }
}

Testing

• ✅ Unit tests pass: ./gradlew :x-pack:plugin:core:test --tests "org.elasticsearch.license.LicenseUtilsTests.testGetDaysUntilExpiry"
• ✅ All existing functionality preserved
• ✅ Backward compatible - no breaking changes

Checklist Response

• Contributor License Agreement: Yes, signed
• Contributor Guidelines: Yes, followed the guidelines in CONTRIBUTING.md
• Built locally: Yes, built and tested with gradle check
• PR against main: Yes, this PR is against the main branch
• Supported OS/Architecture: Yes, developed on macOS which is supported
• Not for class submission: This is an open source contribution, not for academic credit

Resolves #129726