[FEATURE] Add gosdk API wrappers to an interface

Add `gosdk` API wrappers to an interface object. Similar to [TransactionScheme](https://github.com/0chain/gosdk/blob/master/zcncore/transaction.go#L85) but for zboxcore/sdk..

This would help for 3rd party applications, like `zbox` and `zwallet`, that use `gosdk`. They could then use the interface object rather than the raw functions, if nothing else it would make unit tests easier to write.

So an interface object that has as methods at least the `gosdk` API functions used in `zbox` and `zwallet`, and an object that satisfies the interface and wraps the existing functions.

Examine the existing TransactionScheme interface in zcncore/transaction.go and the gosdk API functions used in the zbox and zwallet projects. Create a new interface in zboxcore/sdk that mirrors these functions, then implement a wrapper struct that delegates to the existing raw functions. Verify by checking that the interface can be used in place of the current direct calls in the consumer applications.

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