feat: add retry_policy to synthetics canary

[kushmansingh]

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the library.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Description

Adds support for the new retry policy feature of Canary Synthetics.

Relations

N/A

References

https://aws.amazon.com/about-aws/whats-new/2025/05/aws-cloudwatch-synthetics-canary-updates-automatic-retries/
https://docs.aws.amazon.com/AmazonSynthetics/latest/APIReference/API_CreateCanary.html

Output from Acceptance Testing

% make testacc TESTS=TestAccSyntheticsCanary_retryConfig PKG=synthetics

...

[github-actions]

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[github-actions]

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