API Gateway MTLS "clientCert" Availability?

*Serverless Java Container version*: `eg. 1.5`
1.6

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

*Framework version:* `eg SpringBoot 2.2.6.RELEASE`
2.6.6

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

## Scenario
I am utilizing MTLS authentication for my API Gateway: https://aws.amazon.com/blogs/compute/introducing-mutual-tls-authentication-for-amazon-api-gateway/

In the documentation, it mentions that the `clientCert` is populated in the API GW's event payload when it is passed along to Lambda authorizers via the `requestContext`.

I want to base my authorization on the clientCert's subjectDN's CN within my java code, but I am not sure if it's possible or how I can access the `clientCert` from within my Java lambda utilizing aws-serverless-java-container. (I realize the lambda isn't a `lambda authorizer`, but I was hoping it may still be accessible) Any suggestions?

## Expected behavior
I would expect that `clientCert` is available within `AwsProxyRequest`'s `AwsProxyRequestContext`
EX:
```"requestContext": {
"authentication": {
"clientCert": {
"clientCertPem": "-----BEGIN CERTIFICATE-----\nMIIEZTCCAk0CAQEwDQ...",
"issuerDN": "C=US,ST=Washington,L=Seattle,O=Amazon Web Services,OU=Security,CN=My Private CA",
"serialNumber": "1",
"subjectDN": "C=US,ST=Washington,L=Seattle,O=Amazon Web Services,OU=Security,CN=My Client",
"validity": {
"notAfter": "Aug 5 00:28:21 2120 GMT",
"notBefore": "Aug 29 00:28:21 2020 GMT"
}
}
},
...
```

## Actual behavior
I don't see anything in `AwsProxyRequest` or `AwsProxyRequestContext` related to certs or authentication.

## Steps to reproduce
Set up a REST API Gateway pointing to your `aws-serverless-java-container` lambda. Add custom domain. Create certs. Enable MTLS. Make call to the REST API Gateway and attempt to find `clientCert` object from API GW event payload.

## Full log output
N/A

Commencez par inspecter AwsProxyRequest et AwsProxyRequestContext, puis comparez leurs champs disponibles avec le payload d’événement API Gateway présenté dans l’issue. Vérifiez le chemin de requête MTLS de REST API Gateway et déterminez si les données clientCert parviennent au conteneur Java. Le travail est considéré comme terminé lorsque le contexte de requête pris en charge expose les informations du certificat ou documente clairement qu’elles ne sont pas disponibles.

Rédigé par le modèle d'indexation à partir du texte de l'issue.