PMM-13045 Add RDS assume role support

[HanCheo]

PMM-13045

Link to the Feature Build: Percona-Lab/pmm-submodules#4357 (SUBMODULES-4357)

Summary

• Add optional AWS IAM role ARN support for RDS discovery and RDS exporter configuration.
• Use provided access/secret keys as source credentials when present, otherwise use the AWS SDK default credential chain for local/IRSA credentials.
• Preflight credential resolution so STS AssumeRole/source credential failures surface as explicit InvalidArgument errors instead of generic discovery timeouts.
• Regenerate API artifacts with the AddRDS aws_role_arn field assigned to field number 36 to avoid the existing connection_timeout = 7 field.

API documentation

• API Docs updated

Related work

• JIRA: https://perconadev.atlassian.net/browse/PMM-13045
• Frontend PR: PMM-13045 Add RDS assume role field grafana#901
• Feature Build PR: PMM-13045 RDS assume role feature build Percona-Lab/pmm-submodules#4357

Testing

• go test ./managed/services/management -run 'TestRDSService/DiscoverRDS' -count=1
• git diff --check HEAD~1..HEAD
• Local PMM API call with missing/unauthorized source role now returns explicit STS AccessDenied instead of Request timeout.

[it-percona-cla]

All committers have signed the CLA.