feat: IAM Auth for RDS (#3559)

Have not been yet fully test e2e, but at least this creates the user
with right auth mechanism.

Merging this in will help testing, as I won't need to keep manual docker
images in sync.

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

backend/controller/leases/testdata/go/leases/go.mod

@@ -29,9 +29,19 @@ require (
 	github.com/alecthomas/types v0.17.0 // indirect
 	github.com/amacneil/dbmate/v2 v2.23.0 // indirect
 	github.com/aws/aws-sdk-go-v2 v1.32.5 // indirect
+	github.com/aws/aws-sdk-go-v2/config v1.28.5 // indirect
+	github.com/aws/aws-sdk-go-v2/credentials v1.17.46 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.16.20 // indirect
+	github.com/aws/aws-sdk-go-v2/feature/rds/auth v1.4.24 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/configsources v1.3.24 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.6.24 // indirect
+	github.com/aws/aws-sdk-go-v2/internal/ini v1.8.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/accept-encoding v1.12.1 // indirect
+	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.12.5 // indirect
 	github.com/aws/aws-sdk-go-v2/service/secretsmanager v1.34.6 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sso v1.24.6 // indirect
+	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.28.5 // indirect
+	github.com/aws/aws-sdk-go-v2/service/sts v1.33.1 // indirect
 	github.com/aws/smithy-go v1.22.1 // indirect
 	github.com/benbjohnson/clock v1.3.5 // indirect
 	github.com/block/scaffolder v1.3.0 // indirect