API/Core: Make ScanReport and its related classes Immutable

* Use true immutable objects that are type-safe, thread-safe, null-safe
* Get builder classes for free

This is relying on https://immutables.github.io/ (Apache License 2.0), which allows generating immutable objects and builders via annotation processing.
* Immutable objects are serialization ready (including JSON and its binary forms)
* Supports lazy, derived and optional attributes
* Immutable objects are constructed once, in a consistent state, and can be safely shared
  * Will fail if mandatory attributes are missing
  * Cannot be sneakily modified when passed to other code
* Immutable objects are naturally thread-safe and can therefore be safely shared among threads
  * No excessive copying
  * No excessive synchronization
* Object definitions are pleasant to write and read
  * No boilerplate setter and getters
  * No ugly IDE-generated hashCode, equals and toString methods that end up being stored in source control.

Note that we are specifically preventing people from using Jackson-related annotations (`@JsonSerialize` & `@JsonDeserialize`) in order to avoid potential runtime library dependency issues where a clashing jackson lib would prevent jackson-related annotations to be used/processed.

.baseline/checkstyle/checkstyle.xml

@@ -383,6 +383,11 @@
             <property name="format" value="@Test\(.*expected.*\)"/>
             <property name="message" value="Prefer using Assertions.assertThatThrownBy(...).isInstanceOf(...) instead."/>
         </module>
+        <module name="RegexpSinglelineJava">
+            <property name="ignoreComments" value="true"/>
+            <property name="format" value="@Json(S|Des)erialize"/>
+            <property name="message" value="Avoid using Jackson-related ser-de annotations"/>
+        </module>
         <module name="SimplifyBooleanExpression"/> <!-- Java Coding Guidelines: Keep Boolean expressions simple -->
         <module name="SimplifyBooleanReturn"/> <!-- Java Coding Guidelines: Keep Boolean expressions simple -->
         <module name="StaticVariableName"/> <!-- Java Style Guide: Naming -->