AWS CodeCommit events sent from EventBridge are not received

This issue is mainly to address the generalization of the `Detail` available in a given [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/cloudwatch_events.go) and some duplicate code that I noticed.

**Is your feature request related to a problem? Please describe.**
The current [AWS Codecommit types](https://github.com/aws/aws-lambda-go/blob/master/events/code_commit.go) do not match events sent from EventBridge, because they are made for a direct CodeCommit to Lambda integration. [There are ***24*** unique events](https://docs.aws.amazon.com/codecommit/latest/userguide/monitoring-events.html) that can be sent from an EventBridge CodeCommit rule to a source Lambda function. The events take the structure of [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/cloudwatch_events.go), but the `Detail`s of the events vary.

For example, given this EventBridge event rule (all events from CodeCommit in a particular AWS region):
```json
{
"source": [
"aws.codecommit"
]
}
```

...and this Lambda function signature, similarly documented [here](https://github.com/aws/aws-lambda-go/blob/master/events/README_CodeCommit.md):
```go
func SomeHandler(evt events.CodeCommitEvent) error {
// Handler logic here
return nil
}
```

None of the records are correctly unmarshalled because they do not match this type (again since this is for a direct CodeCommit to Lambda trigger).

**Describe the solution you'd like**
I would consider adding the correct types to encapsulate the data sent from the EventBridge CodeCommit rules after they are matched by [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/cloudwatch_events.go). I don't know what the best way to do this would be since I can't find any documentation on what the behavior is for certain events. For example, if I push ten commits to my CodeCommit repo, will those commits be encapsulated within one event's details or will EventBridge fire ten different events to my function?

I would also consider adding documentation to direct customers to use [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/cloudwatch_events.go) when working with events from EventBridge. This would be more of a temporary solution.

I imagine the best solution scales to ALL Eventbridge source rules, not just CodeCommit.

Also, it looks like there was a [related change](https://github.com/aws/aws-lambda-go/pull/23/commits/92bc2dd27103fafa8ca67723d98ead994a2b87f0) to add Autoscaling related events as well. [`events.AutoScalingEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/autoscaling.go) matches the structure of [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/cloudwatch_events.go) almost exactly, so I'm not sure what happened there. The detail is also being accessed as a `map[string]string` and this could have been accomplished with [`events.CloudWatchEvent`](https://github.com/aws/aws-lambda-go/blob/master/events/code_commit.go) where `Detail` is a `json.RawMessage`.

The most ideal solution also prevents manually adding types for ***every*** service that integrates with Events (EventBridge/CloudWatch Events).

**Describe alternatives you've considered**
I've considered creating all of these structs and switching on the current CloudWatchEvent, but this would take a lot of development overhead. Making a separate package for these types, or the details returned by EventBridge AWS Service events, could become a lot of work.

Generating structures for the event details based on some sort of model or schema may also be a possibility.

**Additional context**
For context, I'm creating a Lambda function that posts CodeCommit events to a [Discord](https://discordapp.com/) server via webhook. Running a switch statement on the 24 types of events is not the issue, but structuring the details of those 24 events is.