Add APIGATEWAY as S3EventSource

[HannesBBR]

One of the supported flows for storing API Gateway Access Logs, is by sending logs to an S3 bucket via Firehose Delivery Streams (https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_apigateway.FirehoseLogDestination.html). The name of the delivery stream always needs to start with amazon-apigateway- (https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-logging-to-kinesis.html), which means the object keys in S3 will also have this string.

Currently, the forwarder doesn't support API Gateway as an S3EventSource, which means the host and service for the logs will be set to s3.

What does this PR do?

Adding API GW as a valid event source for logs stored in S3.

Motivation

Having API GW logs properly parsed if they are stored in S3 (through Kinesis Firehose)

Testing Guidelines

Built my branch locally, and pushed the changes to an existing Forwarder Lambda (using the steps described in the readme). Sending the same S3 event for an API GW log, now correctly sets the source to apigateway.

Additional Notes

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
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[ge0Aja]

lgtm