fix: expose vaadin.eagerServerLoad in Spring properties metadata (#24168) (CP: 25.0) (#24174)

This PR cherry-picks changes from the original PR #24168 to branch 25.0.
---
#### Original PR description
> Add eagerServerLoad to VaadinConfigurationProperties so Spring Boot
can generate configuration metadata and IDE autocomplete for the
documented vaadin.eagerServerLoad property.
> 
> Made-with: Cursor
> 
> <!-- PLEASE READ AND FOLLOW THE TEMPLATE! THE PR CAN BE REJECTED
OTHERWISE (This line should be removed when submitting) -->
> 
> ## Description
> 
> Please list all relevant dependencies in this section and provide
summary of the change, motivation and context.
> 
> Fixes # (issue)
> 
> ## 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.
> 
> #### Additional for `Feature` type of change
> 
> - [ ] Enhancement / new feature was discussed in a corresponding
GitHub issue and Acceptance Criteria were created.
>

Co-authored-by: Peter Großmann <petergrossmann@email.de>
Co-authored-by: Mikhail Shabarov <61410877+mshabarov@users.noreply.github.com>

Author: 3 people

vaadin-spring/src/main/java/com/vaadin/flow/spring/VaadinConfigurationProperties.java

@@ -144,6 +144,12 @@ public static List<String> getAllowedPackages(Environment environment) {
      */
     private int launchBrowserDelay = 30;
 
+    /**
+     * Enables the client-side bootstrap page to include the initial JSON data
+     * fragment.
+     */
+    private boolean eagerServerLoad = false;
+
     /**
      * URL patterns that should not be handled by the Vaadin servlet when mapped
      * to the context root.
@@ -444,6 +450,29 @@ public void setLaunchBrowserDelay(int launchBrowser) {
         this.launchBrowserDelay = launchBrowser;
     }
 
+    /**
+     * Returns whether the client-side bootstrap page should include the initial
+     * JSON data fragment.
+     *
+     * @return {@code true} if eager server load is enabled, {@code false}
+     *         otherwise
+     */
+    public boolean isEagerServerLoad() {
+        return eagerServerLoad;
+    }
+
+    /**
+     * Sets whether the client-side bootstrap page should include the initial
+     * JSON data fragment.
+     *
+     * @param eagerServerLoad
+     *            {@code true} to enable eager server load, {@code false}
+     *            otherwise
+     */
+    public void setEagerServerLoad(boolean eagerServerLoad) {
+        this.eagerServerLoad = eagerServerLoad;
+    }
+
     /**
      * Returns whether class scan caching between reloads when using Spring Boot
      * DevTools should be enabled.