Add JsonPointer annotation for nested binding

[arnabnandy7]

Summary

Adds a new @JsonPointer annotation for selecting a nested JSON value during property deserialization using JSON Pointer syntax.

Part of #357.

Example

Given:

{
  "employee": {
    "details": {
      "departmentId": 123
    }
  }
}

A property can declare its source using:

public class Employee {
    @JsonPointer("/employee/details/departmentId")
    public int departmentId;
}

Changes

• Add the @JsonPointer annotation.
• Support fields, methods, parameters, and bundled annotations.
• Document RFC 6901 escaping rules.
• Define initial semantics:
  • The annotation affects deserialization only.
  • A missing pointer target is treated as an absent property.
  • An explicit JSON null is handled as a normal null property value.
• Add reflection tests for annotation targets and runtime retention.
• Update the 2.23 release notes.

Implementation note

I've defined the annotation and its semantics only. Actual JSON Pointer traversal and property binding implemented separately in jackson-databind in FasterXML/jackson-databind#6161

Testing

./mvnw -Dtest=JsonPointerTest test