feat(lambda-event-sources): starting position timestamp for kafka (#31439)

### Issue # (if applicable)

Closes #31808

### Reason for this change



It was impossible to start consuming a Kafka topic from a specific point in time.

### Description of changes



The user may now set `startingPositionTimestamp` to `ManagedKafkaEventSource` and `SelfManagedKafkaEventSource` to start consuming a Kafka topic from a specific point in time. 

[Lambda and CloudFormation have supported the functionality for a while](https://aws.amazon.com/about-aws/whats-new/2023/06/aws-lambda-starting-timestamp-kafka-sources/), and other stream event sources like `KinesisEventSource` already had a CDK implementation for it. So, technically, this change is doing nothing new; it is only repeating the pattern that has already been proven to work on other sources.

### Description of how you validated changes



The change is tested by adding similar unit tests to other event sources supporting this functionality.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: nikovirtala

packages/@aws-cdk-testing/framework-integ/test/aws-lambda-event-sources/test/integ.kafka-selfmanaged.js.snapshot/LambdaEventSourceKafkaSelfManagedTestDefaultTestDeployAssertAF78BD0F.assets.json

@@ -454,6 +454,128 @@
      "my-test-topic3"
     ]
    }
+  },
+  "F4ServiceRole100FF901": {
+   "Type": "AWS::IAM::Role",
+   "Properties": {
+    "AssumeRolePolicyDocument": {
+     "Statement": [
+      {
+       "Action": "sts:AssumeRole",
+       "Effect": "Allow",
+       "Principal": {
+        "Service": "lambda.amazonaws.com"
+       }
+      }
+     ],
+     "Version": "2012-10-17"
+    },
+    "ManagedPolicyArns": [
+     {
+      "Fn::Join": [
+       "",
+       [
+        "arn:",
+        {
+         "Ref": "AWS::Partition"
+        },
+        ":iam::aws:policy/service-role/AWSLambdaBasicExecutionRole"
+       ]
+      ]
+     }
+    ]
+   }
+  },
+  "F4ServiceRoleDefaultPolicy1E98EC08": {
+   "Type": "AWS::IAM::Policy",
+   "Properties": {
+    "PolicyDocument": {
+     "Statement": [
+      {
+       "Action": [
+        "secretsmanager:DescribeSecret",
+        "secretsmanager:GetSecretValue"
+       ],
+       "Effect": "Allow",
+       "Resource": [
+        {
+         "Ref": "S509448A1"
+        },
+        {
+         "Ref": "SC0855C491"
+        }
+       ]
+      }
+     ],
+     "Version": "2012-10-17"
+    },
+    "PolicyName": "F4ServiceRoleDefaultPolicy1E98EC08",
+    "Roles": [
+     {
+      "Ref": "F4ServiceRole100FF901"
+     }
+    ]
+   }
+  },
+  "F4F1740A13": {
+   "Type": "AWS::Lambda::Function",
+   "Properties": {
+    "Code": {
+     "ZipFile": "exports.handler = async function handler(event) {\n    console.log('event:', JSON.stringify(event, undefined, 2));\n    return { event };\n}"
+    },
+    "Handler": "index.handler",
+    "Role": {
+     "Fn::GetAtt": [
+      "F4ServiceRole100FF901",
+      "Arn"
+     ]
+    },
+    "Runtime": "nodejs18.x"
+   },
+   "DependsOn": [
+    "F4ServiceRoleDefaultPolicy1E98EC08",
+    "F4ServiceRole100FF901"
+   ]
+  },
+  "F4KafkaEventSource838c4d5ff3c99c1a617120adfca83e5bmytesttopic4F6589D62": {
+   "Type": "AWS::Lambda::EventSourceMapping",
+   "Properties": {
+    "BatchSize": 100,
+    "FunctionName": {
+     "Ref": "F4F1740A13"
+    },
+    "SelfManagedEventSource": {
+     "Endpoints": {
+      "KafkaBootstrapServers": [
+       "my-self-hosted-kafka-broker-1:9092",
+       "my-self-hosted-kafka-broker-2:9092",
+       "my-self-hosted-kafka-broker-3:9092"
+      ]
+     }
+    },
+    "SelfManagedKafkaEventSourceConfig": {
+     "ConsumerGroupId": "myTestConsumerGroup4"
+    },
+    "SourceAccessConfigurations": [
+     {
+      "Type": "CLIENT_CERTIFICATE_TLS_AUTH",
+      "URI": {
+       "Ref": "SC0855C491"
+      }
+     },
+     {
+      "Type": "SERVER_ROOT_CA_CERTIFICATE",
+      "URI": {
+       "Ref": "S509448A1"
+      }
+     }
+    ],
+    "StartingPosition": "AT_TIMESTAMP",
+    "StartingPositionTimestamp": 1730270400,
+    "Topics": [
+     "my-test-topic4"
+    ]
+   }
   }
  },
  "Parameters": {