feat: add configurable backend session expiration policy

[mcollovati]

Description

Allows to define an expiration policy for the backend stored session. By default no expiration is set, but can be expiration can be activated setting the vaadin.kubernetes.backend-session-expiration-tolerance property or by providing a custom SessionExpirationPolicy bean.

Fixes #126

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.