feat: Add UploadHandler to Upload

[mshabarov]

Description

Add the new UploadHandler to Upload so with the new API the upload can be done using pre-made implementations, e.g.:

Upload upload = new Upload(UploadHandler.inMemory((meta, data) -> {
    Notifiation.show("Got " + data.length + " bytes for " + meta.getFileName());
  }).onProgress(
    (transferredBytes, totalBytes) -> Notification.show("Received "  + transferredBytes),
    32768 // progress interval in bytes
));

Fixes vaadin/flow#21256

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/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.
• I have not completed some of the steps above and my pull request can be closed immediately.

Additional for Feature type of change

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

[caalador]

Seems ok. Perhaps someone from the components side will have a look also.

[mshabarov]

Requested a review from @yuriy-fix , feel free to reassign to anyone else from your team, thanks in advance 😉

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.8.0.alpha7 and is also targeting the upcoming stable 24.8.0 version.