feat: add generator plugin for endpoints returning signals

[taefi]

Description

The generator-plugin-signals enables the generator
to transform implementation of generated endpoints
containing methods that return a signal instances in
a way to create and return the correct signal instance
instead of Promise.

The added plugin ensures that an endpoint containing
a mixture of methods returning signal instances and
other supported types has correct implementation for
each method. It also ensures the implementation of
endpoints without any methods returning a signal
remains unchanged.

Fixes #2628

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.

[codecov]

Codecov Report

Attention: Patch coverage is 99.19355% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.92%. Comparing base (bb6a2e4) to head (61b182c).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...ts/generator-plugin-signals/src/SignalProcessor.ts	99.19%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2639      +/-   ##
==========================================
+ Coverage   92.58%   92.92%   +0.34%     
==========================================
  Files          71       72       +1     
  Lines        2266     2390     +124     
  Branches      591      611      +20     
==========================================
+ Hits         2098     2221     +123     
- Misses        121      122       +1     
  Partials       47       47              

Flag	Coverage Δ
unittests	92.92% <99.19%> (+0.34%)	⬆️

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[Lodin]

LGTM overall but there is some places to fix

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.9% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

This ticket/PR has been released with Hilla 24.5.0.alpha8 and is also targeting the upcoming stable 24.5.0 version.