Add SSM Session Manager IAM role policy attachment to EC2 RDS DBM Agent Terraform

[swanson8r]

What does this PR do? What is the motivation?

Allows AWS Session Manager Connection to Terraform configured EC2 Host for RDS DBM Agent.

Resource is copied from dd-database-monitoring-example used in https://docs.datadoghq.com/database_monitoring/setup_agent_terraform/

Merge readiness

• Ready for merge

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AI assistance

Additional notes

[rtrieu]

minor suggestion for header but otherwise looks good, thank you for your contribution to our docs!

[jhgilbert]

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