Trim managed agent reason + add retries for getting instance identity signature

[danehlim]

Summary

Add two enhancements to ECS client:

1. Trim managed agent reason to a max of 255 characters
   • This is not strictly required (since currently in practice Agent will not send a reason that exceeds this max), but is more of a safeguard in case Agent sends larger managed agent reasons in the future. For reference, we already have similar trimming logic in place for other "reason" fields such as container reason - we will reuse this logic by defining a reusable function
2. Add retries with exponential backoff for retrieving instance identity signature
   • This is a proactive measure in case we run into any intermittent upstream issues when retrieving instance identity signature. For reference, we already have similar retry with exponential backoff logic for retrieving instance identity document

Implementation details

Same as above.

Testing

New tests cover the changes: yes (existing test also modified and addresses testing bug)

Description for the changelog

Trim managed agent reason + add retries for getting instance identity signature

Does this PR include breaking model changes? If so, Have you added transformation functions?

No

Licensing

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