feat: extend Callables class to let client pass in a custom retry algorithm

[mutianf]

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Fixes #<issue_number_goes_here> ☕️

[sonarqubecloud[bot]]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions

0.0% Coverage on New Code (required ≥ 80%)

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[sonarqubecloud[bot]]

Quality Gate failed for 'java_showcase_integration_tests'

Failed conditions

B Maintainability Rating on New Code (required ≥ A)

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