Need ability to specify AWS Region for Pinpoint

Revisiting a feature request from #817, but with additional context.

Cognito User Pools allows customers to [send auth events to a Pinpoint project](https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-pools-pinpoint-integration.html). However, I found out that it only allows a customer to send events to projects in the `us-east-1` region, regardless of the region in which the User Pool resides.

In order to have amplify CLI manage a Pinpoint project used to receive auth events, the options I see are:

1. Create all resources in `us-east-1`
1. Manually edit the `awsconfiguration.json` file to add the Pinpoint project configuration (which they have to do anyway to add the Pinpoint appId to the config. This would be persisted across `amplify push` invocations, but the project would not be a fully-fledged managed resource.
1. Add a CLI feature allowing the customer to override a region for their Pinpoint project during creation, or afterwards with a configure
1. Add a CLI feature allowing the customer to import a manually-created Pinpoint project into their amplify backend, where the resource may be in a different region than the "main" CFn stack

This is tangentially related to #513, which mapped services to available regions; any feature developed from this request would have to consider service availability as well.