Export AWS partitions and regions as Go constants

[ewbankkit]

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Description

AWS SDK for Go v1 exports AWS partition and region names as constants in https://github.com/aws/aws-sdk-go/blob/main/aws/endpoints/defaults.go. AWS SDK for Go v2 does not export these names.
With the deprecation of AWS SDK for Go v1 and the extensive use of these constants in the Terraform AWS Provider there is the need to define them somewhere.
As these values are already in use in internal/endpoints/endpoints.go it seems to make sense to export these values as constants from this module.

[ewbankkit]

We could maybe generate from https://github.com/aws/aws-sdk-go-v2/blob/main/codegen/smithy-aws-go-codegen/src/main/resources/software/amazon/smithy/aws/go/codegen/endpoints.json?

[gdavison]

Related: #120