Issue description:

It's a known issue that RDS managed credential could generate a credential with special symbols like: XXX$$XXX. When we pass it to the Sample App via terraform template, it could cause string interpolation in bash and the credential got translated to XXX<PID>XXX which will be incorrect.

Description of changes:

This change convert RDS credentials to base64 first and then decode it in the Python Sample App. Re-run Python Sample App Image build workflow is required after merging this PR.

We have verified this solution in Java: #244

Rollback procedure:

<Can we safely revert this commit if needed? If not, detail what must be done to safely revert and why it is needed.>

Yes.

We need to re-run Python Sample App Image build workflow as well after revert.

Ensure you've run the following tests on your changes and include the link below:

Example execution: https://github.com/jerry-shao/aws-application-signals-test-framework/actions/runs/11014255259/job/30584477033

To do so, create a test.yml file with name: Test and workflow description to test your changes, then remove the file for your PR. Link your test run in your PR description. This process is a short term solution while we work on creating a staging environment for testing.

NOTE: TESTS RUNNING ON A SINGLE EKS CLUSTER CANNOT BE RUN IN PARALLEL. See the needs keyword to run tests in succession.

• Run Java EKS on e2e-playground in us-east-1 and eu-central-2
• Run Python EKS on e2e-playground in us-east-1 and eu-central-2
• Run metric limiter on EKS cluster e2e-playground in us-east-1 and eu-central-2
• Run EC2 tests in all regions
• Run K8s on a separate K8s cluster (check IAD test account for master node endpoints; these will change as we create and destroy clusters for OS patching)

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