add: type definitions for customRetry handler

[MikePresman]

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Checklist

• [x] run npm run test and npm run benchmark
• [x] tests and/or benchmarks are included
• [x] documentation is changed or added
• [x] commit message and code follows the Developer's Certification of Origin and the Code of conduct

[mcollina]

type tests are failing

[airhorns]

closing in favour of #337