Implement credential cache in keychain for credential_process

[lyoung-confluent]

Describe your changes

Implements a keychain backed cache for session credentials when using credential_process to invoke granted so GetRoleCredentials is not invoked every time. More details in issue #395.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

Issue and Documentation

395

Testing

Please describe how the reviewer can test the changes. Also include steps to reproduce the testing environment.

1. Configure granted as a credential_process in your ~/.aws/config
2. Invoke granted credential-process --profile demo | jq -r .Expiration
3. Invoke the command again, observe the same token/expiration is returned
4. Invoke the command again adding --window 24h which demonstrates that the cache will be replaced when expiration - window is reached.

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [ ] Do analytics need to be implemented?
• [ ] I have made corresponding changes to the documentation, docs PR has been linked.
• [ ] New and existing unit tests pass with my changes.
• [ ] Any dependent changes have been merged and published in downstream modules.

[sherifabdlnaby]

Thank you for adding this! ❤️