fix: scope AWS config cache per region/profile

[fishdan]

Bug

Terraformer cached a single global aws.Config, so after importing global services (e.g., IAM) the same config was reused for regional passes.
When RDS ran next, it reused the global resolver and hit the wrong host instead of the region-specific rds.us-east-1.amazonaws.com, causing imports to fail.

Fix

Replace the singleton config pointer with a mutex-protected map keyed by region/profile/skip flags.
Build a fresh aws.Config for each unique key and cache it so the RDS pass resolves the correct regional endpoint while still reusing configs within the same scope.

Testing

gofmt providers/aws/aws_service.go
~/go/bin/go test ./providers/aws
Manual terraformer import aws --resources=rds --regions=us-east-1 (followed by other services) now succeeds against the standard RDS endpoint.

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