Use @types/indy-sdk to enforce typing of methods

[TimoGlastra]

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Suggestion to drop flow and move to TS. We could depend on @types/indy-sdk and make sure the export in this repo matches the Indy type from @types/indy-sdk.

This way we also don't have the problem anymore with @types/rn-indy-sdk because it will have types out of the box