feat: add theme API to MessageListItem

[pekam]

NOTE: This depends on #978 and won't pass in CI before that is merged.

Description

Feature: add and remove theme attribute values to messages in a MessageList. Only with strings at this point, since no theme variants have been implemented so far. The API is copied from HasTheme interface, which can be directly implemented only by component classes.

This enables developers to apply custom theming per each message in a MessageList.

Related issue: vaadin/collaboration-engine#30

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.

[vaadin-bot]

SonarQube analysis reported 2 issues

1. MessageListItem.java#L450: Make this anonymous inner class a lambda
2. MessageListItem.java#L54: Make this value-based field transient so it is not included in the serialization of this class.

[vaadin-bot]

This ticket/PR has been released with platform 21.0.0.alpha2. For prerelease versions, it will be included in its final version.