Add inline policy recommendation for secret management

[tbarizien]

Issue:

When only using AmazonRDSFullAccess and setting a DBInstance spec.manageMasterUserPassword to true, the controller would fail on DBInstanceCreate because of insufficient permissions (first to use KMS keys then to create passwords) .

This allows the controller to create and manage secrets in secrets manager.

Description of changes:

Adds a recommended-inline-policy file containing the necessary policy for the RDS controller to manage secrets in the AWS SecretStore.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-prow]

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This pull-request has been approved by: tbarizien
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[ack-prow]

Hi @tbarizien. Thanks for your PR.

I'm waiting for a aws-controllers-k8s member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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