CodePipelineEvent hardcoded to CodePipelineJob.

**Is your feature request related to a problem? Please describe.**

`CodePipelineEvent` seems to be hardcoded as a `CodePipelineJob`
- https://github.com/aws/aws-lambda-go/blob/master/events/codepipeline_job.go#L6-L8

This incorrectly makes CodePipeline event unusable for its actual events.

[CodePipelineEvent documentation](CodePipelineEvent)

**Describe the solution you'd like**

I would like to move the current `CodePipelineEvent` into its own `CodePipelineJobEvent` as it seems to be seperate then the documented CodePipelineEvents

**Describe alternatives you've considered**

Considered just leaving it as is and creating `CodePipelineEvent` under some new name like `ActualCodePipelineEvent` but I think this is one of those instances where it would be better to just make the change.

**Additional context**

`Breaking Change`