feat: i18n translation request backend integration

[ugur-vaadin]

Description

This PR:

• implements default backend integration for translation requests
• adds an i18n Spring test project
• adds ITs for the test project

Fixes #2103

Type of change

• [ ] Bugfix
• [x] Feature

Checklist

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

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.78%. Comparing base (def33e6) to head (522a929).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2168   +/-   ##
=======================================
  Coverage   93.77%   93.78%           
=======================================
  Files          64       64           
  Lines        1622     1624    +2     
  Branches      364      365    +1     
=======================================
+ Hits         1521     1523    +2     
  Misses         66       66           
  Partials       35       35           

Flag	Coverage Δ
unittests	93.78% <100.00%> (+<0.01%)	:arrow_up:

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[sonarqubecloud[bot]]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.3% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

This ticket/PR has been released with Hilla 24.4.0.alpha14 and is also targeting the upcoming stable 24.4.0 version.