Provide concrete example of building custom events

*To help us debug your issue fill in the basic information below using the options provided*

*Serverless Java Container version*: `eg. 1.5`
v1 (Spring Boot 2) and v2 (Spring Boot 3.x)

*Implementations:* `Jersey / Spring / Spring Boot / Spring Boot 2 / Spark`
Spring Boot

*Framework version:* `eg SpringBoot 2.2.6.RELEASE`
Spring Boot 2.x and Spring Boot 3.x

*Frontend service:* `REST API / HTTP API / ALB`
VPC Lattice

*Deployment method:* `eg SAM, Serverless Framework, Console`
N/A

## Scenario
*Describe what you are trying to accomplish*
This is a more general ask related to this [issue](https://github.com/awslabs/aws-serverless-java-container/issues/639). The documentation provided [here](https://github.com/awslabs/aws-serverless-java-container/wiki/Custom-event-types) indicates that just four classes need to be implemented to build the custom class. However, pursuant of trying to implement the VPC Lattice event, I discovered that in addition to creating the POJOs for the event model itself, I needed to also implement `ExceptionHandler`, `SecurityContext`, `SecurityContextWriter`. It would helpful to include a practical example of how to create the custom event, outline the minimum and optional attributes for the servlet request, and how to set fields for the response payload.

## Expected behavior
*Describe how you would expect the application to behave*
Ability to simply or at least clear extend the framework with custom event payloads

## Actual behavior
*Describe what you are seeing instead*
Difficulty following the documentation