Add Pinpoint Events

**Is your feature request related to a problem? Please describe.**
We are receiving Pinpoint events from Kinesis, but it seems there are no official-ish types for them anywhere except for the documentation below.

* https://docs.aws.amazon.com/pinpoint/latest/developerguide/event-streams-data-journey.html
* https://docs.aws.amazon.com/pinpoint/latest/developerguide/event-streams-data-email.html
* https://docs.aws.amazon.com/pinpoint/latest/developerguide/event-streams-data-sms.html

**Describe the solution you'd like**
`events.PinpointEvent` or `events.PinpointEmailEvent`, `events.PinpointSMSEvent`, `events.PinpointJourneyEvent` if they can't be expressed as a single type.

**Describe alternatives you've considered**
I've looked everywhere for a definition of these events, but haven't found anything yet.
We are currently using custom types to parse these, but it would be nice to have them here, or even better in the SDK itself.

**Additional context**
Add any other context or screenshots about the feature request here.

Start with the three AWS Pinpoint event-stream documentation links in the issue and compare their schemas to determine whether one shared event type is suitable. The issue names no repository files or tests; done means adding official Go event types that represent the documented email, SMS, and journey events.

Written by the indexing model from the issue text.